Web Servers Implementation and Performance Erich Nahum IBM

Web Servers: Implementation and Performance Erich Nahum IBM T. J. Watson Research Center www. research. ibm. com/people/n/nahum@us. ibm. com Web Servers: Implementation and Performance Erich Nahum 1

Contents and Timeline: • Introduction to the Web (30 min): – HTTP, Clients, Servers, Proxies, DNS, CDN’s • Outline of a Web Server Transaction (25 min): – Receiving a request, generating a response • Web Server Architectural Models (20 min): – Processes, threads, events • Web Server Workload Characteristics (30 min): – File sizes, document popularity, embedded objects • Web Server Workload Generation (20 min): – Webstone, Spec. Web, TPC-W Web Servers: Implementation and Performance Erich Nahum 2

Things Not Covered in Tutorial • Client-side issues: HTML rendering, Javascript interpretation • TCP issues: implementation, interaction with HTTP • Proxies: some similarities, many differences • Dynamic Content: CGI, PHP, EJB, ASP, etc. • Qo. S for Web Servers • SSL/TLS and HTTPS • Content Distribution Networks (CDN’s) • Security and Denial of Service Web Servers: Implementation and Performance Erich Nahum 3

Assumptions and Expectations • Some familiarity with WWW as a user (Has anyone here not used a browser? ) • Some familiarity with networking concepts (e. g. , unreliability, reordering, race conditions) • Familiarity with systems programming (e. g. , know what sockets, hashing, caching are) • Examples will be based on C & Unix taken from BSD, Linux, AIX, and real servers (sorry, Java and Windows fans) Web Servers: Implementation and Performance Erich Nahum 4

Objectives and Takeaways After this tutorial, hopefully we will all know: • • • Basics of the Web, HTTP, clients, servers, DNS Basics of server implementation & performance Pros and cons of various server architectures Characteristics of web server workloads Difficulties in workload generation Design loop of implement, measure, debug, and fix Many lessons should be applicable to any networked server, e. g. , files, mail, news, DNS, LDAP, etc. Web Servers: Implementation and Performance Erich Nahum 5

Acknowledgements Many people contributed comments and suggestions to this tutorial, including: Abhishek Chandra Mark Crovella Suresh Chari Peter Druschel Jim Kurose Balachander Krishnamurthy Vivek Pai Jennifer Rexford Anees Shaikh Srinivasan Seshan Errors are all mine, of course. Web Servers: Implementation and Performance Erich Nahum 6

Chapter 1: Introduction to the World-Wide Web (WWW) Web Servers: Implementation and Performance Erich Nahum 7

Introduction to the WWW http request http response Client Proxy Server • HTTP: Hypertext Transfer Protocol – Communication protocol between clients and servers – Application layer protocol for WWW • Client/Server model: – Client: browser that requests, receives, displays object – Server: receives requests and responds to them – Proxy: intermediary that aggregates requests, responses • Protocol consists of various operations – Few for HTTP 1. 0 (RFC 1945, 1996) – Many more in HTTP 1. 1 (RFC 2616, 1999) Web Servers: Implementation and Performance Erich Nahum 8

How are Requests Generated? • User clicks on something • Uniform Resource Locator (URL): – – – http: //www. nytimes. com https: //www. paymybills. com ftp: //ftp. kernel. org news: //news. deja. com telnet: //gaia. cs. umass. edu mailto: nahum@us. ibm. com • Different URL schemes map to different services • Hostname is converted from a name to a 32 -bit IP address (DNS resolve) • Connection is established to server Most browser requests are HTTP requests. Web Servers: Implementation and Performance Erich Nahum 9

How are DNS names resolved? • Clients have a well-known IP address for a local DNS name server • Clients ask local name server for IP address • Local name server may not know it, however! • Name server has, in turn, a parent to ask (the “DNS hierarchy”) • The local name server’s job is to iteratively query servers until name is found and return IP address to server • Each name server can cache names, but: – Each name: IP mapping has a time-to-live field – After time expires, name server must discard mapping Web Servers: Implementation and Performance Erich Nahum 10

DNS in Action myclient. watson. ibm. com 200 OK index. html GET /index. html www. ipam. ucla. edu 12. 100. 104. 5 www. ipam. ucla. edu? 12. 100. 104. 5 (TTL = 10 min) ns. watson. ibm. com (name server) www. ipam. ucla. edu? ns. ucl ww w. ip a. ed am u (T . uc TL =1 d) A. GTLD-SERVER. NET (name server for. edu) la. e du? Web Servers: Implementation and Performance ns. ucla. edu (name server) Erich Nahum 11

What Happens Then? • Client downloads HTML document – Sometimes called “container page” – Typically in text format (ASCII) – Contains instructions for rendering (e. g. , background color, frames) – Links to other pages • Many have embedded objects: – Images: GIF, JPG (logos, banner ads) – Usually automatically retrieved • I. e. , without user involvement • can control sometimes (e. g. browser options, junkbuster) Web Servers: Implementation and Performance Erich Nahum <html> <head> <meta name=“Author” content=“Erich Nahum”> <title> Linux Web Server Performance </title> </head> <body text=“#00000”> <img width=31 height=11 src=“ibmlogo. gif”> <img src=“images/new. gif> <h 1>Hi There!</h 1> Here’s lots of cool linux stuff! <a href=“more. html”> Click here</a> for more! </body> </html> sample html file 12

So What’s a Web Server Do? • Respond to client requests, typically a browser – Can be a proxy, which aggregates client requests (e. g. , AOL) – Could be search engine spider or custom (e. g. , Keynote) • May have work to do on client’s behalf: – – Is the client’s cached copy still good? Is client authorized to get this document? Is client a proxy on someone else’s behalf? Run an arbitrary program (e. g. , stock trade) • Hundreds or thousands of simultaneous clients • Hard to predict how many will show up on some day • Many requests are in progress concurrently Server capacity planning is non-trivial. Web Servers: Implementation and Performance Erich Nahum 13

What do HTTP Requests Look Like? GET /images/penguin. gif HTTP/1. 0 User-Agent: Mozilla/0. 9. 4 (Linux 2. 2. 19) Host: www. kernel. org Accept: text/html, image/gif, image/jpeg Accept-Encoding: gzip Accept-Language: en Accept-Charset: iso-8859 -1, *, utf-8 Cookie: B=xh 203 jfsf; Y=3 sdkfjej <cr><lf> • Messages are in ASCII (human-readable) • Carriage-return and line-feed indicate end of headers • Headers may communicate private information (browser, OS, cookie information, etc. ) Web Servers: Implementation and Performance Erich Nahum 14

What Kind of Requests are there? Called Methods: • GET: retrieve a file (95% of requests) • HEAD: just get meta-data (e. g. , mod time) • POST: submitting a form to a server • PUT: store enclosed document as URI • DELETE: removed named resource • LINK/UNLINK: in 1. 0, gone in 1. 1 • TRACE: http “echo” for debugging (added in 1. 1) • CONNECT: used by proxies for tunneling (1. 1) • OPTIONS: request for server/proxy options (1. 1) Web Servers: Implementation and Performance Erich Nahum 15

What Do Responses Look Like? HTTP/1. 0 200 OK Server: Tux 2. 0 Content-Type: image/gif Content-Length: 43 Last-Modified: Fri, 15 Apr 1994 02: 36: 21 GMT Expires: Wed, 20 Feb 2002 18: 54: 46 GMT Date: Mon, 12 Nov 2001 14: 29: 48 GMT Cache-Control: no-cache Pragma: no-cache Connection: close Set-Cookie: PA=wefj 2 we 0 -jfjf <cr><lf> <data follows…> • Similar format to requests (i. e. , ASCII) Web Servers: Implementation and Performance Erich Nahum 16

What Responses are There? • 1 XX: Informational (def’d in 1. 0, used in 1. 1) 100 Continue, 101 Switching Protocols • 2 XX: Success 200 OK, 206 Partial Content • 3 XX: Redirection 301 Moved Permanently, 304 Not Modified • 4 XX: Client error 400 Bad Request, 403 Forbidden, 404 Not Found • 5 XX: Server error 500 Internal Server Error, 503 Service Unavailable, 505 HTTP Version Not Supported Web Servers: Implementation and Performance Erich Nahum 17

What are all these Headers? Specify capabilities and properties: • General: Connection, Date • Request: Accept-Encoding, User-Agent • Response: Location, Server type • Entity: Content-Encoding, Last-Modified • Hop-by-hop: Proxy-Authenticate, Transfer-Encoding Server must pay attention to respond properly. Web Servers: Implementation and Performance Erich Nahum 18

The Role of Proxies clients Internet proxy servers • Clients send requests to local proxy • Proxy sends requests to remote servers • Proxy can cache responses and return them Web Servers: Implementation and Performance Erich Nahum 19

Why have a Proxy? • For performance: – Many of the same web documents are requested by many different clients (“locality of reference”) – A copy of the document can be cached for later requests (typical document hit rate: ~ 50%) – Since proxy is closer to client, responses times are smaller than from server • For cost savings: – Organizations pay by ISP bandwidth used – Cached responses don’t consume ISP bandwidth • For security/policy: – Typically located in “demilitarized zone” (DMZ) – Easier to protect a single point rather than all clients – Can enforce corporate/government policies (e. g. , porn) Web Servers: Implementation and Performance Erich Nahum 20

Proxy Placement in the Web proxy clients Internet proxy “reverse” proxy servers proxy • Proxies can be placed in arbitrary points in net: – Can be organized into hierarchies – Placed in front of a server: “reverse” proxy – Route requests to specific proxies: content distribution Web Servers: Implementation and Performance Erich Nahum 21

Content Distribution Networks proxy origin servers Internet proxy • Push content out to proxies: – Route client requests to “closest” proxy – Reduce load on origin server – Reduce response time seen by client Web Servers: Implementation and Performance Erich Nahum 22

Mechanisms for CDN’s • IP Anycast: – Route an IP packet to one-of-many IP addresses – Some research but not deployed or supported by IPV 4 • TCP Redirection: – Client TCP packets go to one machine, but responses come from a different one – Clunky, not clear it reduces load or response time • HTTP Redirection: – When client connects, use 302 response (moved temp) to send client to proxy close to client – Server must be aware of CDN network • DNS Redirection: – When client asks for server IP address, tell them based on where they are in the network – Used by most CDN providers (e. g. , Akamai) Web Servers: Implementation and Performance Erich Nahum 23

DNS Based Request-Routing cdn 1 cdn 2 www. service. com cdn 3 cdn 5 cdn 4 service. com? client service. com? cdn 3 requestrouting DNS name server local nameserver Web Servers: Implementation and Performance Erich Nahum 24

Summary: Introduction to WWW • The major application on the Internet – Majority of traffic is HTTP (or HTTP-related) – Messages mostly in ASCII text (helps debugging!) • Client/server model: – Clients make requests, servers respond to them – Proxies act as servers to clients, clients to servers • Content may be spread across network – Through either proxy caches or content distr. networks – DNS redirection is the common approach to CDNs • Various HTTP headers and commands – Too many to go into detail here – We’ll focus on common server ones – Many web books/tutorials exist (e. g. , Krishnamurthy & Rexford 2001) Web Servers: Implementation and Performance Erich Nahum 25

Chapter 2: Outline of a Typical Web Server Transaction Web Servers: Implementation and Performance Erich Nahum 26

Outline of an HTTP Transaction • In this section we go over the basics of servicing an HTTP GET request from user space • For this example, we'll assume a single process running in user space, similar to Apache 1. 3 • At each stage see what the costs/problems can be • Also try to think of where costs can be optimized • We’ll describe relevant socket operations as we go Web Servers: Implementation and Performance Erich Nahum initialize; forever do { get request; process; send response; log request; } server in a nutshell 27

Readying a Server s = socket(); bind(s, 80); listen(s); while (1) { newconn = /* allocate listen socket */ /* bind to TCP port 80 */ /* indicate willingness to accept */ accept(s); /* accept new connection */b • First thing a server does is notify the OS it is interested in WWW server requests; these are typically on TCP port 80. Other services use different ports (e. g. , SSL is on 443) • Allocate a socket and bind()'s it to the address (port 80) • Server calls listen() on the socket to indicate willingness to receive requests • Calls accept() to wait for a request to come in (and blocks) • When the accept() returns, we have a new socket which represents a new connection to a client Web Servers: Implementation and Performance Erich Nahum 28

Processing a Request remote. IP = getsockname(newconn); remote. Host = gethostbyname(remote. IP); gettimeofday(current. Time); read(newconn, req. Buffer, sizeof(req. Buffer)); req. Info = server. Parse(req. Buffer); • getsockname() called to get the remote host name • gethostbyname() called to get name of other end • gettimeofday() is called to get time of request • • read() is called on new socket to retrieve request is determined by parsing the data – for logging purposes (optional, but done by most) – again for logging purposes – both for Date header and for logging – “GET /images/jul 4/flag. gif” Web Servers: Implementation and Performance Erich Nahum 29

Processing a Request (cont) file. Name = parse. Out. File. Name(request. Buffer); file. Attr = stat(file. Name); server. Check. File. Stuff(file. Name, file. Attr); open(file. Name); • stat() called to test file path – to see if file exists/is accessible – may not be there, may only be available to certain people – "/microsoft/top-secret/plans-for-world-domination. html" • stat() also used for file meta-data – e. g. , size of file, last modified time – "Have plans changed since last time I checked? “ • might have to stat() multiple files just to get to end – e. g. , 4 stats in bill g example above • assuming all is OK, open() called to open the file Web Servers: Implementation and Performance Erich Nahum 30

Responding to a Request read(file. Name, file. Buffer); header. Buffer = server. Figure. Headers(file. Name, req. Info); write(new. Sock, header. Buffer); write(new. Sock, file. Buffer); close(new. Sock); close(file. Name); write(log. File, request. Info); • • read() called to read the file into user space write() is called to send HTTP headers on socket (early servers called write() for each header!) • • write() is called to write the file on the socket close() is called to close the open file descriptor write() is called on the log file Web Servers: Implementation and Performance Erich Nahum 31

Optimizing the Basic Structure • As we will see, a great deal of locality exists in web requests and web traffic. • Much of the work described above doesn't really need to be performed each time. • Optimizations fall under 2 categories: caching and custom OS primitives. Web Servers: Implementation and Performance Erich Nahum 32

Optimizations: Caching Idea is to exploit locality in client requests. Many files are requested over and over (e. g. , index. html). • Why open and close files over and over again? Instead, cache open file FD’s, manage them LRU. file. Descriptor = look. In. FDCache(file. Name); meta. Info = look. In. Meta. Info. Cache(file. Name); header. Buffer = look. In. HTTPHeader. Cache(file. Name); • Why stat them again • Again, cache HTTP header and again? Cache path info on a per-url basis, name and access rather than re-generating characteristics. info over and over. Web Servers: Implementation and Performance Erich Nahum 33

Optimizations: Caching (cont) • Instead of reading and writing the data, cache data, as well as meta-data, in user space • Even better, mmap() the file so that two copies don't exist in both user and kernel space file. Data = look. In. File. Data. Cache(file. Name); file. Data = look. In. MMap. Cache(file. Name); remote. Host. Name = look. Remote. Host. Cache(file. Name); • Since we see the same clients over and over, cache the reverse name lookups (or better yet, don't do resolves at all, log only IP addresses) Web Servers: Implementation and Performance Erich Nahum 34

Optimizations: OS Primitives • Rather than call accept(), getsockname() & read(), add a new primitive, accept. Extended(), which combines the 3 primitives • Instead of calling gettimeofday(), use a memory-mapped counter that is cheap to access (a few instructions rather than a system call) accept. Extended(listen. Sock, &new. Sock, read. Buffer, &remote. Info); current. Time = *mapped. Time. Pointer; buffer[0] = first. HTTPHeader; buffer[1] = second. HTTPHeader; buffer[2] = file. Data. Buffer; writev(new. Sock, buffer, 3); • Instead of calling write() many times, use writev() Web Servers: Implementation and Performance Erich Nahum 35

OS Primitives (cont) • Rather than calling read() & write(), or write() with an mmap()'ed file, use a new primitive called sendfile() (or transmitfile()). Bytes stay in the kernel. • While we're at it, add a header option to sendfile() so that we don't have to call write() at all. http. Info = cache. Lookup(req. Buffer); sendfile(new. Conn, http. Info->headers, http. Info->file. Descriptor, OPT_CLOSE_WHEN_DONE); • Also add an option to close the connection so that we don't have to call close() explicitly. All this assumes proper OS support. Most have it these days. Web Servers: Implementation and Performance Erich Nahum 36

An Accelerated Server Example acceptex(socket, new. Conn, req. Buffer, remote. Host. Info); http. Info = cache. Lookup(req. Buffer); sendfile(new. Conn, http. Info->headers, http. Info->file. Descriptor, OPT_CLOSE_WHEN_DONE); write(log. File, request. Info); • acceptex() is called – gets new socket, request, remote host IP address • string match in hash table is done to parse request – hash table entry contains relevant meta-data, including modification times, file descriptors, permissions, etc. • sendfile() is called – pre-computed header, file descriptor, and close option • log written back asynchronously (buffered write()). That’s it! Web Servers: Implementation and Performance Erich Nahum 37

Complications • Much of this assumes sharing is easy: – but, this is dependent on the server architectural model – if multiple processes are being used, as in Apache, it is difficult to share data structures. • Take, for example, mmap(): – mmap() maps a file into the address space of a process. – a file mmap'ed in one address space can’t be re-used for a request for the same file served by another process. – Apache 1. 3 does use mmap() instead of read(). – in this case, mmap() eliminates one data copy versus a separate read() & write() combination, but process will still need to open() and close() the file. Web Servers: Implementation and Performance Erich Nahum 38

Complications (cont) • Similarly, meta-data info needs to be shared: – e. g. , file size, access permissions, last modified time, etc. • While locality is high, cache misses can and do happen sometimes: – if previously unseen file requested, process can block waiting for disk. • OS can impose other restrictions: – e. g. , limits on number of open file descriptors. – e. g. , sockets typically allow buffering about 64 KB of data. If a process tries to write() a 1 MB file, it will block until other end receives the data. • Need to be able to cope with the misses without slowing down the hits Web Servers: Implementation and Performance Erich Nahum 39

Summary: Outline of a Typical HTTP Transaction • A server can perform many steps in the process of servicing a request • Different actions depending on many factors: – e. g. , 304 not modified if client's cached copy is good – e. g. , 404 not found, 401 unauthorized • Most requests are for small subset of data: – we’ll see more about this in the Workload section – we can leverage that fact for performance • Architectural model affects possible optimizations – we’ll go into this in more detail in the next section Web Servers: Implementation and Performance Erich Nahum 40

Chapter 3: Server Architectural Models Web Servers: Implementation and Performance Erich Nahum 41

Server Architectural Models Several approaches to server structure: • Process based: Apache, NCSA • Thread-based: JAWS, IIS • Event-based: Flash, Zeus • Kernel-based: Tux, AFPA, Exo. Kernel We will describe the advantages and disadvantages of each. Fundamental tradeoffs exist between performance, protection, sharing, robustness, extensibility, etc. Web Servers: Implementation and Performance Erich Nahum 42

Process Model (ex: Apache) • Process created to handle each new request: – Process can block on appropriate actions, (e. g. , socket read, file read, socket write) – Concurrency handled via multiple processes • Quickly becomes unwieldy: – Process creation is expensive. – Instead, pre-forked pool is created. – Upper limit on # of processes is enforced • First by the server, eventually by the operating system. • Concurrency is limited by upper bound Web Servers: Implementation and Performance Erich Nahum 43

Process Model: Pros and Cons • Advantages: – Most importantly, consistent with programmer's way of thinking. Most programmers think in terms of linear series of steps to accomplish task. – Processes are protected from one another; can't nuke data in some other address space. Similarly, if one crashes, others unaffected. • Disadvantages: – Slow. Forking is expensive, allocating stack, VM data structures for each process adds up and puts pressure on the memory system. – Difficulty in sharing info across processes. – Have to use locking. – No control over scheduling decisions. Web Servers: Implementation and Performance Erich Nahum 44

Thread Model (Ex: JAWS) • Use threads instead of processes. Threads consume fewer resources than processes (e. g. , stack, VM allocation). • Forking and deleting threads is cheaper than processes. • Similarly, pre-forked thread pool is created. May be limits to numbers but hopefully less of an issue than with processes since fewer resources required. Web Servers: Implementation and Performance Erich Nahum 45

Thread Model: Pros and Cons • Advantages: – Faster than processes. Creating/destroying cheaper. – Maintains programmer's way of thinking. – Sharing is enabled by default. • Disadvantages: – Less robust. Threads not protected from each other. – Requires proper OS support, otherwise, if one thread blocks on a file read, will block all the address space. – Can still run out of threads if servicing many clients concurrently. – Can exhaust certain per-process limits not encountered with processes (e. g. , number of open file descriptors). – Limited or no control over scheduling decisions. Web Servers: Implementation and Performance Erich Nahum 46

Event Model (Ex: Flash) while (1) { accept new connections until none remaining; call select() on all active file descriptors; for each FD: if (fd ready for reading) call read(); if (fd ready for writing) call write(); } • Use a single process and deal with requests in a event-driven manner, like a giant switchboard. • Use non-blocking option (O_NDELAY) on sockets, do everything asynchronously, never block on anything, and have OS notify us when something is ready. Web Servers: Implementation and Performance Erich Nahum 47

Event-Driven: Pros and Cons • Advantages: – – – Very fast. Sharing is inherent, since there’s only one process. Don't even need locks as in thread models. Can maximize concurrency in request stream easily. No context-switch costs or extra memory consumption. Complete control over scheduling decisions. • Disadvantages: – Less robust. Failure can halt whole server. – Pushes per-process resource limits (like file descriptors). – Not every OS has full asynchronous I/O, so can still block on a file read. Flash uses helper processes to deal with this (AMPED architecture). Web Servers: Implementation and Performance Erich Nahum 48

In-Kernel Model (Ex: Tux) HTTP SOCK user/ kernel boundary HTTP user/ kernel boundary TCP IP IP ETH user-space server kernel-space server • Dedicated kernel thread for HTTP requests: – One option: put whole server in kernel. – More likely, just deal with static GET requests in kernel to capture majority of requests. – Punt dynamic requests to full-scale server in user space, such as Apache. Web Servers: Implementation and Performance Erich Nahum 49

In-Kernel Model: Pros and Cons • In-kernel event model: – Avoids transitions to user space, copies across u-k boundary, etc. – Leverages already existing asynchronous primitives in the kernel (kernel doesn't block on a file read, etc. ) • Advantages: – Extremely fast. Tight integration with kernel. – Small component without full server optimizes common case. • Disadvantages: – Less robust. Bugs can crash whole machine, not just server. – Harder to debug and extend, since kernel programming required, which is not as well-known as sockets. – Similarly, harder to deploy. APIs are OS-specific (Linux, BSD, NT), whereas sockets & threads are (mostly) standardized. – HTTP evolving over time, have to modify kernel code in response. Web Servers: Implementation and Performance Erich Nahum 50

So What’s the Performance? • Graph shows server throughput for Tux, Flash, and Apache. • Experiments done on 400 MHz P/II, gigabit Ethernet, Linux 2. 4. 9 -ac 10, 8 client machines, Wasp. Client workload generator • Tux is fastest, but Flash close behind Web Servers: Implementation and Performance Erich Nahum 51

Summary: Server Architectures • Many ways to code up a server – Tradeoffs in speed, safety, robustness, ease of programming and extensibility, etc. • Multiple servers exist for each kind of model – Not clear that a consensus exists. • Better case for in-kernel servers as devices e. g. reverse proxy accelerator, Akamai CDN node • User-space servers have a role: – OS should provides proper primitives for efficiency – Leave HTTP-protocol related actions in user-space – In this case, event-driven model is attractive • Key pieces to a fast event-driven server: – Minimize copying – Efficient event notification mechanism Web Servers: Implementation and Performance Erich Nahum 52

Chapter 5: Workload Characterization Web Servers: Implementation and Performance Erich Nahum 53

Workload Characterization • Why Characterize Workloads? – Gives an idea about traffic behavior ("Which documents are users interested in? ") – Aids in capacity planning ("Is the number of clients increasing over time? ") – Aids in implementation ("Does caching help? ") • How do we capture them ? – Through server logs (typically enabled) – Through packet traces (harder to obtain and to process) Web Servers: Implementation and Performance Erich Nahum 54

Factors to Consider client? proxy? server? • Where do I get logs from? – Client logs give us an idea, but not necessarily the same – Same for proxy logs – What we care about is the workload at the server • Is trace representative? – Corporate POP vs. News vs. Shopping site • What kind of time resolution? – e. g. , second, millisecond, microsecond • Does trace/log capture all the traffic? – e. g. , incoming link only, or one node out of a cluster Web Servers: Implementation and Performance Erich Nahum 55

Probability Refresher • Lots of variability in workloads • Some terminology/jargon: • Heavy-tailed: – – – Use probability distributions to express Want to consider many factors Mean: average of samples Median : half are bigger, half are smaller Percentiles: dump samples into N bins (median is 50 th percentile number) As x->infinity Web Servers: Implementation and Performance Erich Nahum 56

Important Distributions Some Frequently-Seen Distributions: • Normal: • Lognormal: • Exponential: • Pareto: – – (avg. sigma, variance mu) (x >= 0; sigma > 0) (x >= k, shape a, scale k) Web Servers: Implementation and Performance Erich Nahum 57

More Probability • Graph shows 3 distributions with average = 2. • Note average median in some cases ! • Different distributions have different “weight” in tail. Web Servers: Implementation and Performance Erich Nahum 58

What Info is Useful? • Request methods – GET, POST, HEAD, etc. • Response codes • • • – success, failure, not-modified, etc. Size of requested files Size of transferred objects Popularity of requested files Numbers of embedded objects Inter-arrival time between requests Protocol support (1. 0 vs. 1. 1) Web Servers: Implementation and Performance Erich Nahum 59

Sample Logs for Illustration Name: Chess 1997 Olympics 1998 IBM 2001 Description: Kasparov. Deep Blue Event Site Nagano 1998 Olympics Event Site Corporate Presence Period: 2 weeks in May 1997 2 days in Feb 1998 1 day in June 1998 1 day in Feb 2001 Hits: 1, 586, 667 5, 800, 000 11, 485, 600 12, 445, 739 Bytes: 14, 171, 711 10, 515, 507 54, 697, 108 28, 804, 852 Clients: 256, 382 80, 921 86, 0211 319, 698 URLS: 2, 293 30, 465 15, 788 42, 874 We’ll use statistics generated from these logs as examples. Web Servers: Implementation and Performance Erich Nahum 60

Request Methods GET Chess 1997 96% Olympics IBM 1998 99. 6% 99. 3% HEAD 04% 00. 3 % POST 00. 007% 00. 04 % Others: noise IBM 2001 97% 00. 08% 02% 00. 2% noise • KR 01: "overwhelming majority" are GETs, few POSTs • IBM 2001 trace starts seeing a few 1. 1 methods (CONNECT, OPTIONS, LINK), but still very small (1/10^5 %) Web Servers: Implementation and Performance Erich Nahum 61

Response Codes Code Meaning Chess 1997 Olympics 1998 IBM 2001 200 204 206 301 302 304 400 401 403 404 407 500 501 503 ? ? ? OK NO_CONTENT PARTIAL_CONTENT MOVED_PERMANENTLY MOVED_TEMPORARILY NOT_MODIFIED BAD_REQUEST UNAUTHORIZED FORBIDDEN NOT_FOUND PROXY_AUTH SERVER_ERROR NOT_IMPLEMENTED SERVICE_UNAVAIL UNKNOWN 85. 32 --. -00. 25 00. 05 13. 73 00. 001 --. — 00. 01 00. 55 --. ---. -00. 0003 76. 02 --. ---. -00. 05 23. 24 00. 0001 00. 02 00. 64 --. -00. 003 00. 0001 --. -00. 00004 75. 28 00. 00001 --. -01. 18 22. 84 00. 003 00. 0001 00. 65 --. -00. 006 00. 0005 00. 0001 00. 005 67. 72 --. ---. -15. 11 16. 26 00. 001 00. 009 00. 79 00. 002 00. 07 00. 006 00. 0003 00. 0004 • Table shows percentage of responses. • Majority are OK and NOT_MODIFIED. • Consistent with numbers from AW 96, KR 01. Web Servers: Implementation and Performance Erich Nahum 62

Resource (File) Sizes • • • Shows file/memory usage (not weighted by frequency!) Lognormal body, consistent with results from AW 96, CB 96, KR 01. AW 96, CB 96: sizes have Pareto tail; Downey 01: Sizes are lognormal. Web Servers: Implementation and Performance Erich Nahum 63

Tails from the File Size • Shows the complementary CDF (CCDF) of file sizes. • Haven’t done the curve fitting but looks Pareto-ish. Web Servers: Implementation and Performance Erich Nahum 64

Response (Transfer) Sizes • Shows network usage (weighted by frequency of requests) • Lognormal body, pareto tail, consistent with CBC 95, AW 96, CB 96, KR 01 Web Servers: Implementation and Performance Erich Nahum 65

Tails of Transfer Size • Shows the complementary CDF (CCDF) of file sizes. • Looks more Pareto-like; certainly some big transfers. Web Servers: Implementation and Performance Erich Nahum 66

Resource Popularity • Follows a Zipf model: p(r) = r^{-alpha} • • Consistent with CBC 95, AW 96, CB 96, PQ 00, KR 01 Shows that caching popular documents is very effective (alpha = 1 true Zipf; others “Zipf-like") Web Servers: Implementation and Performance Erich Nahum 67

Number of Embedded Objects • Mah 97: avg 3, 90% are 5 or less • BC 98: pareto distr, median 0. 8, mean 1. 7 • Arlitt 98 World Cup study: median 15 objects, 90% are 20 or less • MW 00: median 7 -17, mean 11 -18, 90% 40 or less • STA 00: median 5, 30 (2 traces), 90% 50 or less • Mah 97, BC 98, SCJO 01: embedded objects tend to be smaller than container objects • KR 01: median is 8 -20, pareto distribution Trend seems to be that number is increasing over time. Web Servers: Implementation and Performance Erich Nahum 68

Session Inter-Arrivals • Inter-arrival time between successive requests – “Think time" – difference between user requests vs. ALL requests – partly depends on definition of boundary • CB 96: variability across multiple timescales, "selfsimilarity", average load very different from peak or heavy load • SCJO 01: log-normal, 90% less than 1 minute. • AW 96: independent and exponentially distributed • KR 01: pareto with a=1. 5, session arrivals follow poisson distribution, but requests follow pareto Web Servers: Implementation and Performance Erich Nahum 69

Protocol Support • IBM. com 2001 logs: – Show roughly 53% of client requests are 1. 1 • KA 01 study: – 92% of servers claim to support 1. 1 (as of Sep 00) – Only 31% actually do; most fail to comply with spec • SCJO 01 show: – Avg 6. 5 requests persistent connection – 65% have 2 connections per page, rest more. – 40 -50% of objects downloaded by persistent connections Appears that we are in the middle of a slow transition to 1. 1 Web Servers: Implementation and Performance Erich Nahum 70

Summary: Workload Characterization • Traffic is variable: – Responses vary across multiple orders of magnitude • Traffic is bursty: – Peak loads much larger than average loads • Certain files more popular than others – Zipf-like distribution captures this well • Two-sided aspect of transfers: – Most responses are small (zero pretty common) – Most of the bytes are from large transfers • Controversy over Pareto/log-normal distribution • Non-trivial for workload generators to replicate Web Servers: Implementation and Performance Erich Nahum 71

Chapter 6: Workload Generators Web Servers: Implementation and Performance Erich Nahum 72

Why Workload Generators? • Allows stress-testing and bug -finding • Gives us some idea of server capacity • Allows us a scientific process to compare approaches – e. g. , server models, gigabit adaptors, OS implementations • Assumption is that difference in testbed translates to some difference in real-world • Allows the performance debugging cycle Web Servers: Implementation and Performance Measure Fix and/or improve Reproduce Find Problem The Performance Debugging Cycle Erich Nahum 73

Problems with Workload Generators • Only as good as our understanding of the traffic • Traffic may change over time – generators must too • May not be representative – e. g. , are file size distributions from IBM. com similar to mine? • May be ignoring important factors – e. g. , browser behavior, WAN conditions, modem connectivity • Still, useful for diagnosing and treating problems Web Servers: Implementation and Performance Erich Nahum 74

How does W. Generation Work? • Many clients, one server – match asymmetry of Internet • Server is populated with some kind of synthetic content • Simulated clients produce requests for server • Master process to control clients, aggregate results • Goal is to measure server – not the client or network Requests Responses • Must be robust to conditions – e. g. , if server keeps sending 404 not found, will clients notice? Web Servers: Implementation and Performance Erich Nahum 75

Evolution: Web. Stone • • • The original workload generator from SGI in 1995 Process based workload generator, implemented in C Clients talk to master via sockets Configurable: # client machines, # client processes, run time Measured several metrics: avg + max connect time, response time, throughput rate (bits/sec), # pages, # files • 1. 0 only does GETS, CGI support added in 2. 0 • Static requests, 5 different file sizes: Percentage Size 35. 00 500 B 50. 00 5 KB 14. 00 50 KB 0. 90 500 KB 0. 10 5 MB Web Servers: Implementation and Performance www. mindcraft. com/webstone Erich Nahum 76

Evolution: SPECWeb 96 • Developed by SPEC – Systems Performance Evaluation Consortium – Non-profit group with many benchmarks (CPU, FS) • Attempt to get more representative – Based on logs from NCSA, HP, Hal Computers • 4 classes of files: Percentage Size 35. 00 0 -1 KB 50. 00 1 -10 KB 14. 00 10 -100 KB 1. 00 100 KB – 1 MB • Poisson distribution between each class Web Servers: Implementation and Performance Erich Nahum 77

SPECWeb 96 (cont) • Notion of scaling versus load: – number of directories in data set size doubles as expected throughput quadruples (sqrt(throughput/5)*10) – requests spread evenly across all application directories • Process based WG • Clients talk to master via RPC's (less robust) • Still only does GETS, no keep-alive www. spec. org/osg/web 96 Web Servers: Implementation and Performance Erich Nahum 78

Evolution: SURGE • Scalable URL Reference GEnerator – Barford & Crovella at Boston University CS Dept. • Much more worried about representativeness, captures: – – – server file size distributions, request size distribution, relative file popularity embedded file references temporal locality of reference idle periods ("think times") of users • Process/thread based WG Web Servers: Implementation and Performance Erich Nahum 79

SURGE (cont) • Notion of “user-equivalent”: – statistical model of a user – active “off” time (between URLS), – inactive “off” time (between pages) • Captures various levels of burstiness • Not validated, shows that load generated is different than Spec. Web 96 and has more burstiness in terms of CPU and # active connections www. cs. wisc. edu/~pb Web Servers: Implementation and Performance Erich Nahum 80

Evolution: S-client • Almost all workload generators are closed-loop: – client submits a request, waits for server, maybe thinks for some time, repeat as necessary • Problem with the closed-loop approach: – client can't generate requests faster than the server can respond – limits the generated load to the capacity of the server – in the real world, arrivals don’t depend on server state • i. e. , real users have no idea about load on the server when they click on a site, although successive clicks may have this property – in particular, can't overload the server • s-client tries to be open-loop: – by generating connections at a particular rate – independent of server load/capacity Web Servers: Implementation and Performance Erich Nahum 81

S-Client (cont) • How is s-client open-loop? – connecting asynchronously at a particular rate – using non-blocking connect() socket call • Connect complete within a particular time? – if yes, continue normally. – if not, socket is closed and new connect initiated. • Other details: – uses single-address space event-driven model like Flash – calls select() on large numbers of file descriptors – can generate large loads • Problems: – client capacity is still limited by active FD's – “arrival” is a TCP connect, not an HTTP request www. cs. rice. edu/CS/Systems/Web-measurement Web Servers: Implementation and Performance Erich Nahum 82

Evolution: SPECWeb 99 • In response to people "gaming" benchmark, now includes rules: – IP maximum segment lifetime (MSL) must be at least 60 seconds (more on this later!) – Link-layer maximum transmission unit (MTU) must not be larger than 1460 bytes (Ethernet frame size) – Dynamic content may not be cached • not clear that this is followed – Servers must log requests. • W 3 C common log format is sufficient but not mandatory. – Resulting workload must be within 10% of target. – Error rate must be below 1%. • Metric has changed: – now "number of simultaneous conforming connections“: rate of a connection must be greater than 320 Kbps Web Servers: Implementation and Performance Erich Nahum 83

SPECWeb 99 (cont) • Directory size has changed: (25 + (400000/122000)* simultaneous conns) / 5. 0) • Improved HTTP 1. 0/1. 1 support: – Keep-alive requests (client closes after N requests) – Cookies • Back-end notion of user demographics – Used for ad rotation – Request includes user_id and last_ad • Request breakdown: – – – 70. 00 % static GET 12. 45 % dynamic GET 12. 60 % dynamic GET with custom ad rotation 04. 80 % dynamic POST 00. 15 % dynamic GET calling CGI code Web Servers: Implementation and Performance Erich Nahum 84

SPECWeb 99 (cont) • Other breakdowns: – – 30 % HTTP 1. 0 with no keep-alive or persistence 70 % HTTP 1. 0 with keep-alive to "model" persistence still has 4 classes of file size with Poisson distribution supports Zipf popularity • Client implementation details: – Master-client communication now uses sockets – Code includes sample Perl code for CGI – Client configurable to use threads or processes • Much more info on setup, debugging, tuning • All results posted to web page, – including configuration & back end code www. spec. org/osg/web 99 Web Servers: Implementation and Performance Erich Nahum 85

Spec. Web 99 vs. File Sizes • Spec. Web 99: In the ballpark, but not very smooth Web Servers: Implementation and Performance Erich Nahum 86

Spec. Web 99 vs. File Size Tail • Spec. Web 99 tail isn’t as long as real logs (900 KB max) Web Servers: Implementation and Performance Erich Nahum 87

Spec. Web 99 vs. Transfer Sizes • Doesn’t capture 304 (not modified) responses • Coarser distribution than real logs (i. e. , not smooth) Web Servers: Implementation and Performance Erich Nahum 88

Spec 99 vs. Transfer Size Tails • Spec. Web 99 does OK, although tail drops off rapidly (and in fact, no file is greater than 1 MB in Spec. Web 99!). Web Servers: Implementation and Performance Erich Nahum 89

Spec 99 vs. Resource Popularity • Spec. Web 99 seems to do a good job, although tail isn’t long enough Web Servers: Implementation and Performance Erich Nahum 90

Evolution: TPC-W • Transaction Processing Council (TPC-W) – – More known for database workloads like TPC-D Metrics include dollars/transaction (unlike SPEC) Provides specification, not source Meant to capture a large e-commerce site – – – web serving, searching, browsing, shopping carts online transaction processing (OLTP) decision support (DSS) secure purchasing (SSL), best sellers, new products customer registration, administrative updates • Models online bookstore • Has notion of scaling per user – 5 MB of DB tables per user – 1 KB per shopping item, 25 KB per item in static images Web Servers: Implementation and Performance Erich Nahum 91

TPC-W (cont) • Remote browser emulator (RBE) – emulates a single user – send HTTP request, parse, wait for thinking, repeat • Metrics: – WIPS: shopping – WIPSb: browsing – WIPSo: ordering • Setups tend to be very large: – multiple image servers, application servers, load balancer – DB back end (typically SMP) – Example: IBM 12 -way SMP w/DB 2, 9 PCs w/IIS: 1 M $ www. tpc. org/tpcw Web Servers: Implementation and Performance Erich Nahum 92

Summary: Workload Generators • Only the beginning. Many other workload generators: – – httperf from HP WAGON from IBM Wasp. Client from IBM Others? • Both workloads and generators change over time: – Both started simple, got more complex – As workload changes, so must generators • No one single "good" generator – Spec. Web 99 seems the favorite (2002 rumored in the works) • Implementation issues similar to servers: – They are networked-based request producers (i. e. , produce GET's instead of 200 OK's). – Implementation affects capacity planning of clients! (want to make sure clients are not bottleneck) Web Servers: Implementation and Performance Erich Nahum 93

End of this tutorial… • This is roughly half of a four-hour tutorial: – ACM SIGMETRICS 2002 (June, Marina Del Ray, CA) • Remainder gets into more detailed issues: – – Event notification mechanisms in servers Overview of the TCP protocol TCP dynamics for servers TCP implementation issues for servers • Talk to me if you’re still interested, or • Point your browser at: www. sigmetrics. org Web Servers: Implementation and Performance Erich Nahum 94

Chapter: Event Notification • Event notification: – Mechanism for kernel and application to notify each other of interesting/important events – E. g. , connection arrivals, socket closes, data available to read, space available for writing • Idea is to exploit concurrency: – Concurrency in user workloads means host CPU can overlap multiple events to maximize parallelism – Keep network, disk busy; never block • Simultaneously, want to minimize costs: – user/kernel crossings and testing idle socket descriptors • Event notification changes applications: – state-based to event-based – requires a change in thinking Web Servers: Implementation and Performance Erich Nahum 95

Chapter: Introduction to TCP • Layering is a common principle in network protocol design • TCP is the major transport protocol in the Internet • Since HTTP runs on top of TCP, much interaction between the two • Asymmetry in client-server model puts strain on server-side TCP implementations • Thus, major issue in web servers is TCP implementation and behavior Web Servers: Implementation and Performance Erich Nahum application transport network link physical 96

Chapter: TCP Dynamics • In this section we'll describe some of the problems you can run into as a WWW server interacting with TCP. • Most of these affect the response as seen by the client, not the throughput generated by the server. • Ideally, a server developer shouldn't have to worry about this stuff, but in practice, we'll see that's not the case. • Examples we'll look at include: – – The initial window size The delayed ACK problem Nagle and its interaction with delayed ack Small receive windows interfering with loss recovery Web Servers: Implementation and Performance Erich Nahum 97

Chapter: Server TCP Implementation • In this section we look at ways in which the host TCP implementation is stressed under large web server workloads. Most of these techniques deal with large numbers of connections: – Looking up arriving TCP segments with large numbers of connections – Dealing with the TIME-WAIT state caused by closing large number of connections – Managing large numbers of timers to support connections – Dealing with memory consumption of connection state • Removing data-touching operations – byte copying and checksums Web Servers: Implementation and Performance Erich Nahum 98