doc/fcgi-perf.htm

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   5          Understanding FastCGI Application Performance
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  22          <A HREF="http://fastcgi.com"><IMG BORDER="0" SRC="../images/fcgi-hd.gif" ALT="[[FastCGI]]"></A>
  23       </DIV>
  24       <BR CLEAR="all">
  25       <DIV CLASS="c1">
  26          <H3>
  27             Understanding FastCGI Application Performance
  28          </H3>
  29       </DIV>
  30       <!--Copyright (c) 1996 Open Market, Inc.                                    -->
  31       <!--See the file "LICENSE.TERMS" for information on usage and redistribution-->
  32       <!--of this file, and for a DISCLAIMER OF ALL WARRANTIES.                   -->
  33       <DIV CLASS="c1">
  34          Mark R. Brown<BR>
  35          Open Market, Inc.<BR>
  36          <P>
  37             10 June 1996<BR>
  38          </P>
  39       </DIV>
  40       <P>
  41       </P>
  42       <H5 CLASS="c2">
  43          Copyright &copy; 1996 Open Market, Inc. 245 First Street, Cambridge, MA 02142 U.S.A.<BR>
  44          Tel: 617-621-9500 Fax: 617-621-1703 URL: <A HREF=
  45          "http://www.openmarket.com/">http://www.openmarket.com/</A><BR>
  46          $Id: fcgi-perf.htm,v 1.4 2002/02/25 00:42:59 robs Exp $<BR>
  47       </H5>
  48       <HR>
  49       <UL TYPE="square">
  50          <LI>
  51             <A HREF="#S1">1. Introduction</A>
  52          </LI>
  53          <LI>
  54             <A HREF="#S2">2. Performance Basics</A>
  55          </LI>
  56          <LI>
  57             <A HREF="#S3">3. Caching</A>
  58          </LI>
  59          <LI>
  60             <A HREF="#S4">4. Database Access</A>
  61          </LI>
  62          <LI>
  63             <A HREF="#S5">5. A Performance Test</A>
  64             <UL TYPE="square">
  65                <LI>
  66                   <A HREF="#S5.1">5.1 Application Scenario</A>
  67                </LI>
  68                <LI>
  69                   <A HREF="#S5.2">5.2 Application Design</A>
  70                </LI>
  71                <LI>
  72                   <A HREF="#S5.3">5.3 Test Conditions</A>
  73                </LI>
  74                <LI>
  75                   <A HREF="#S5.4">5.4 Test Results and Discussion</A>
  76                </LI>
  77             </UL>
  78          </LI>
  79          <LI>
  80             <A HREF="#S6">6. Multi-threaded APIs</A>
  81          </LI>
  82          <LI>
  83             <A HREF="#S7">7. Conclusion</A>
  84          </LI>
  85       </UL>
  86       <P>
  87       </P>
  88       <HR>
  89       <H3>
  90          <A NAME="S1">1. Introduction</A>
  91       </H3>
  92       <P>
  93          Just how fast is FastCGI? How does the performance of a FastCGI application compare with the performance of
  94          the same application implemented using a Web server API?
  95       </P>
  96       <P>
  97          Of course, the answer is that it depends upon the application. A more complete answer is that FastCGI often
  98          wins by a significant margin, and seldom loses by very much.
  99       </P>
 100       <P>
 101          Papers on computer system performance can be laden with complex graphs showing how this varies with that.
 102          Seldom do the graphs shed much light on <I>why</I> one system is faster than another. Advertising copy is
 103          often even less informative. An ad from one large Web server vendor says that its server &quot;executes web
 104          applications up to five times faster than all other servers,&quot; but the ad gives little clue where the
 105          number &quot;five&quot; came from.
 106       </P>
 107       <P>
 108          This paper is meant to convey an understanding of the primary factors that influence the performance of Web
 109          server applications and to show that architectural differences between FastCGI and server APIs often give an
 110          &quot;unfair&quot; performance advantage to FastCGI applications. We run a test that shows a FastCGI
 111          application running three times faster than the corresponding Web server API application. Under different
 112          conditions this factor might be larger or smaller. We show you what you&#39;d need to measure to figure that
 113          out for the situation you face, rather than just saying &quot;we&#39;re three times faster&quot; and moving
 114          on.
 115       </P>
 116       <P>
 117          This paper makes no attempt to prove that FastCGI is better than Web server APIs for every application. Web
 118          server APIs enable lightweight protocol extensions, such as Open Market&#39;s SecureLink extension, to be
 119          added to Web servers, as well as allowing other forms of server customization. But APIs are not well matched
 120          to mainstream applications such as personalized content or access to corporate databases, because of API
 121          drawbacks including high complexity, low security, and limited scalability. FastCGI shines when used for the
 122          vast majority of Web applications.
 123       </P>
 124       <P>
 125       </P>
 126       <H3>
 127          <A NAME="S2">2. Performance Basics</A>
 128       </H3>
 129       <P>
 130          Since this paper is about performance we need to be clear on what &quot;performance&quot; is.
 131       </P>
 132       <P>
 133          The standard way to measure performance in a request-response system like the Web is to measure peak request
 134          throughput subject to a response time constriaint. For instance, a Web server application might be capable of
 135          performing 20 requests per second while responding to 90% of the requests in less than 2 seconds.
 136       </P>
 137       <P>
 138          Response time is a thorny thing to measure on the Web because client communications links to the Internet have
 139          widely varying bandwidth. If the client is slow to read the server&#39;s response, response time at both the
 140          client and the server will go up, and there&#39;s nothing the server can do about it. For the purposes of
 141          making repeatable measurements the client should have a high-bandwidth communications link to the server.
 142       </P>
 143       <P>
 144          [Footnote: When designing a Web server application that will be accessed over slow (e.g. 14.4 or even 28.8
 145          kilobit/second modem) channels, pay attention to the simultaneous connections bottleneck. Some servers are
 146          limited by design to only 100 or 200 simultaneous connections. If your application sends 50 kilobytes of data
 147          to a typical client that can read 2 kilobytes per second, then a request takes 25 seconds to complete. If your
 148          server is limited to 100 simultaneous connections, throughput is limited to just 4 requests per second.]
 149       </P>
 150       <P>
 151          Response time is seldom an issue when load is light, but response times rise quickly as the system approaches
 152          a bottleneck on some limited resource. The three resources that typical systems run out of are network I/O,
 153          disk I/O, and processor time. If short response time is a goal, it is a good idea to stay at or below 50% load
 154          on each of these resources. For instance, if your disk subsystem is capable of delivering 200 I/Os per second,
 155          then try to run your application at 100 I/Os per second to avoid having the disk subsystem contribute to slow
 156          response times. Through careful management it is possible to succeed in running closer to the edge, but
 157          careful management is both difficult and expensive so few systems get it.
 158       </P>
 159       <P>
 160          If a Web server application is local to the Web server machine, then its internal design has no impact on
 161          network I/O. Application design can have a big impact on usage of disk I/O and processor time.
 162       </P>
 163       <P>
 164       </P>
 165       <H3>
 166          <A NAME="S3">3. Caching</A>
 167       </H3>
 168       <P>
 169          It is a rare Web server application that doesn&#39;t run fast when all the information it needs is available
 170          in its memory. And if the application doesn&#39;t run fast under those conditions, the possible solutions are
 171          evident: Tune the processor-hungry parts of the application, install a faster processor, or change the
 172          application&#39;s functional specification so it doesn&#39;t need to do so much work.
 173       </P>
 174       <P>
 175          The way to make information available in memory is by caching. A cache is an in-memory data structure that
 176          contains information that&#39;s been read from its permanent home on disk. When the application needs
 177          information, it consults the cache, and uses the information if it is there. Otherwise is reads the
 178          information from disk and places a copy in the cache. If the cache is full, the application discards some old
 179          information before adding the new. When the application needs to change cached information, it changes both
 180          the cache entry and the information on disk. That way, if the application crashes, no information is lost; the
 181          application just runs more slowly for awhile after restarting, because the cache doesn&#39;t improve
 182          performance when it is empty.
 183       </P>
 184       <P>
 185          Caching can reduce both disk I/O and processor time, because reading information from disk uses more processor
 186          time than reading it from the cache. Because caching addresses both of the potential bottlenecks, it is the
 187          focal point of high-performance Web server application design. CGI applications couldn&#39;t perform in-memory
 188          caching, because they exited after processing just one request. Web server APIs promised to solve this
 189          problem. But how effective is the solution?
 190       </P>
 191       <P>
 192          Today&#39;s most widely deployed Web server APIs are based on a pool-of-processes server model. The Web server
 193          consists of a parent process and a pool of child processes. Processes do not share memory. An incoming request
 194          is assigned to an idle child at random. The child runs the request to completion before accepting a new
 195          request. A typical server has 32 child processes, a large server has 100 or 200.
 196       </P>
 197       <P>
 198          In-memory caching works very poorly in this server model because processes do not share memory and incoming
 199          requests are assigned to processes at random. For instance, to keep a frequently-used file available in memory
 200          the server must keep a file copy per child, which wastes memory. When the file is modified all the children
 201          need to be notified, which is complex (the APIs don&#39;t provide a way to do it).
 202       </P>
 203       <P>
 204          FastCGI is designed to allow effective in-memory caching. Requests are routed from any child process to a
 205          FastCGI application server. The FastCGI application process maintains an in-memory cache.
 206       </P>
 207       <P>
 208          In some cases a single FastCGI application server won&#39;t provide enough performance. FastCGI provides two
 209          solutions: session affinity and multi-threading.
 210       </P>
 211       <P>
 212          With session affinity you run a pool of application processes and the Web server routes requests to individual
 213          processes based on any information contained in the request. For instance, the server can route according to
 214          the area of content that&#39;s been requested, or according to the user. The user might be identified by an
 215          application-specific session identifier, by the user ID contained in an Open Market Secure Link ticket, by the
 216          Basic Authentication user name, or whatever. Each process maintains its own cache, and session affinity
 217          ensures that each incoming request has access to the cache that will speed up processing the most.
 218       </P>
 219       <P>
 220          With multi-threading you run an application process that is designed to handle several requests at the same
 221          time. The threads handling concurrent requests share process memory, so they all have access to the same
 222          cache. Multi-threaded programming is complex -- concurrency makes programs difficult to test and debug -- but
 223          with FastCGI you can write single threaded <I>or</I> multithreaded applications.
 224       </P>
 225       <P>
 226       </P>
 227       <H3>
 228          <A NAME="S4">4. Database Access</A>
 229       </H3>
 230       <P>
 231          Many Web server applications perform database access. Existing databases contain a lot of valuable
 232          information; Web server applications allow companies to give wider access to the information.
 233       </P>
 234       <P>
 235          Access to database management systems, even within a single machine, is via connection-oriented protocols. An
 236          application &quot;logs in&quot; to a database, creating a connection, then performs one or more accesses.
 237          Frequently, the cost of creating the database connection is several times the cost of accessing data over an
 238          established connection.
 239       </P>
 240       <P>
 241          To a first approximation database connections are just another type of state to be cached in memory by an
 242          application, so the discussion of caching above applies to caching database connections.
 243       </P>
 244       <P>
 245          But database connections are special in one respect: They are often the basis for database licensing. You pay
 246          the database vendor according to the number of concurrent connections the database system can sustain. A
 247          100-connection license costs much more than a 5-connection license. It follows that caching a database
 248          connection per Web server child process is not just wasteful of system&#39;s hardware resources, it could
 249          break your software budget.
 250       </P>
 251       <P>
 252       </P>
 253       <H3>
 254          <A NAME="S5">5. A Performance Test</A>
 255       </H3>
 256       <P>
 257          We designed a test application to illustrate performance issues. The application represents a class of
 258          applications that deliver personalized content. The test application is quite a bit simpler than any real
 259          application would be, but still illustrates the main performance issues. We implemented the application using
 260          both FastCGI and a current Web server API, and measured the performance of each.
 261       </P>
 262       <P>
 263       </P>
 264       <H4>
 265          <A NAME="S5.1">5.1 Application Scenario</A>
 266       </H4>
 267       <P>
 268          The application is based on a user database and a set of content files. When a user requests a content file,
 269          the application performs substitutions in the file using information from the user database. The application
 270          then returns the modified content to the user.
 271       </P>
 272       <P>
 273          Each request accomplishes the following:
 274       </P>
 275       <P>
 276       </P>
 277       <OL>
 278          <LI>
 279             authentication check: The user id is used to retrieve and check the password.
 280             <P>
 281             </P>
 282          </LI>
 283          <LI>
 284             attribute retrieval: The user id is used to retrieve all of the user&#39;s attribute values.
 285             <P>
 286             </P>
 287          </LI>
 288          <LI>
 289             file retrieval and filtering: The request identifies a content file. This file is read and all occurrences
 290             of variable names are replaced with the user&#39;s corresponding attribute values. The modified HTML is
 291             returned to the user.<BR>
 292             <BR>
 293          </LI>
 294       </OL>
 295       <P>
 296          Of course, it is fair game to perform caching to shortcut any of these steps.
 297       </P>
 298       <P>
 299          Each user&#39;s database record (including password and attribute values) is approximately 100 bytes long.
 300          Each content file is 3,000 bytes long. Both database and content files are stored on disks attached to the
 301          server platform.
 302       </P>
 303       <P>
 304          A typical user makes 10 file accesses with realistic think times (30-60 seconds) between accesses, then
 305          disappears for a long time.
 306       </P>
 307       <P>
 308       </P>
 309       <H4>
 310          <A NAME="S5.2">5.2 Application Design</A>
 311       </H4>
 312       <P>
 313          The FastCGI application maintains a cache of recently-accessed attribute values from the database. When the
 314          cache misses the application reads from the database. Because only a small number of FastCGI application
 315          processes are needed, each process opens a database connection on startup and keeps it open.
 316       </P>
 317       <P>
 318          The FastCGI application is configured as multiple application processes. This is desirable in order to get
 319          concurrent application processing during database reads and file reads. Requests are routed to these
 320          application processes using FastCGI session affinity keyed on the user id. This way all a user&#39;s requests
 321          after the first hit in the application&#39;s cache.
 322       </P>
 323       <P>
 324          The API application does not maintain a cache; the API application has no way to share the cache among its
 325          processes, so the cache hit rate would be too low to make caching pay. The API application opens and closes a
 326          database connection on every request; keeping database connections open between requests would result in an
 327          unrealistically large number of database connections open at the same time, and very low utilization of each
 328          connection.
 329       </P>
 330       <P>
 331       </P>
 332       <H4>
 333          <A NAME="S5.3">5.3 Test Conditions</A>
 334       </H4>
 335       <P>
 336          The test load is generated by 10 HTTP client processes. The processes represent disjoint sets of users. A
 337          process makes a request for a user, then a request for a different user, and so on until it is time for the
 338          first user to make another request.
 339       </P>
 340       <P>
 341          For simplicity the 10 client processes run on the same machine as the Web server. This avoids the possibility
 342          that a network bottleneck will obscure the test results. The database system also runs on this machine, as
 343          specified in the application scenario.
 344       </P>
 345       <P>
 346          Response time is not an issue under the test conditions. We just measure throughput.
 347       </P>
 348       <P>
 349          The API Web server is in these tests is Netscape 1.1.
 350       </P>
 351       <P>
 352       </P>
 353       <H4>
 354          <A NAME="S5.4">5.4 Test Results and Discussion</A>
 355       </H4>
 356       <P>
 357          Here are the test results:
 358       </P>
 359       <P>
 360       </P>
 361       <DIV CLASS="c3">
 362 <PRE>
 363     FastCGI  12.0 msec per request = 83 requests per second
 364     API      36.6 msec per request = 27 requests per second
 365 </PRE>
 366       </DIV>
 367       <P>
 368          Given the big architectural advantage that the FastCGI application enjoys over the API application, it is not
 369          surprising that the FastCGI application runs a lot faster. To gain a deeper understanding of these results we
 370          measured two more conditions:
 371       </P>
 372       <P>
 373       </P>
 374       <UL>
 375          <LI>
 376             API with sustained database connections. If you could afford the extra licensing cost, how much faster
 377             would your API application run?
 378             <P>
 379             </P>
 380 <PRE>
 381     API      16.0 msec per request = 61 requests per second
 382 </PRE>
 383             Answer: Still not as fast as the FastCGI application.
 384             <P>
 385             </P>
 386          </LI>
 387          <LI>
 388             FastCGI with cache disabled. How much benefit does the FastCGI application get from its cache?
 389             <P>
 390             </P>
 391 <PRE>
 392     FastCGI  20.1 msec per request = 50 requests per second
 393 </PRE>
 394             Answer: A very substantial benefit, even though the database access is quite simple.<BR>
 395             <BR>
 396          </LI>
 397       </UL>
 398       <P>
 399          What these two extra experiments show is that if the API and FastCGI applications are implemented in exactly
 400          the same way -- caching database connections but not caching user profile data -- the API application is
 401          slightly faster. This is what you&#39;d expect, since the FastCGI application has to pay the cost of
 402          inter-process communication not present in the API application.
 403       </P>
 404       <P>
 405          In the real world the two applications would not be implemented in the same way. FastCGI&#39;s architectural
 406          advantage results in much higher performance -- a factor of 3 in this test. With a remote database or more
 407          expensive database access the factor would be higher. With more substantial processing of the content files
 408          the factor would be smaller.
 409       </P>
 410       <P>
 411       </P>
 412       <H3>
 413          <A NAME="S6">6. Multi-threaded APIs</A>
 414       </H3>
 415       <P>
 416          Web servers with a multi-threaded internal structure (and APIs to match) are now starting to become more
 417          common. These servers don&#39;t have all of the disadvantages described in Section 3. Does this mean that
 418          FastCGI&#39;s performance advantages will disappear?
 419       </P>
 420       <P>
 421          A superficial analysis says yes. An API-based application in a single-process, multi-threaded server can
 422          maintain caches and database connections the same way a FastCGI application can. The API-based application
 423          does not pay for inter-process communication, so the API-based application will be slightly faster than the
 424          FastCGI application.
 425       </P>
 426       <P>
 427          A deeper analysis says no. Multi-threaded programming is complex, because concurrency makes programs much more
 428          difficult to test and debug. In the case of multi-threaded programming to Web server APIs, the normal problems
 429          with multi-threading are compounded by the lack of isolation between different applications and between the
 430          applications and the Web server. With FastCGI you can write programs in the familiar single-threaded style,
 431          get all the reliability and maintainability of process isolation, and still get very high performance. If you
 432          truly need multi-threading, you can write multi-threaded FastCGI and still isolate your multi-threaded
 433          application from other applications and from the server. In short, multi-threading makes Web server APIs
 434          unusable for practially all applications, reducing the choice to FastCGI versus CGI. The performance winner in
 435          that contest is obviously FastCGI.
 436       </P>
 437       <P>
 438       </P>
 439       <H3>
 440          <A NAME="S7">7. Conclusion</A>
 441       </H3>
 442       <P>
 443          Just how fast is FastCGI? The answer: very fast indeed. Not because it has some specially-greased path through
 444          the operating system, but because its design is well matched to the needs of most applications. We invite you
 445          to make FastCGI the fast, open foundation for your Web server applications.
 446       </P>
 447       <P>
 448       </P>
 449       <HR>
 450       <A HREF="http://www.openmarket.com/"><IMG SRC="omi-logo.gif" ALT="OMI Home Page"></A>
 451       <ADDRESS>
 452          &copy; 1995, Open Market, Inc. / mbrown@openmarket.com
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