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CS193H: High Performance Web Sites Lecture 8: Rule 4 – Gzip Components

CS193H: High Performance Web Sites Lecture 8: Rule 4 – Gzip Components. Steve Souders Google souders@cs.stanford.edu. Announcements. Web 100 Performance Profile (round 1) class project has been graded – contact Aravind if you want to know your grade. Compression (encoding).

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CS193H: High Performance Web Sites Lecture 8: Rule 4 – Gzip Components

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  1. CS193H:High Performance Web SitesLecture 8: Rule 4 – Gzip Components Steve Souders Google souders@cs.stanford.edu

  2. Announcements Web 100 Performance Profile (round 1) class project has been graded – contact Aravind if you want to know your grade

  3. Compression (encoding) typically reduces size by 70% (6230-2066)/6230 = 67% GET /v-app/scripts/107652916-dom.common.js HTTP/1.1 Host: www.blogger.com User-Agent: Mozilla/5.0 (…) Gecko/2008070208 Firefox/3.0.1 GET /v-app/scripts/107652916-dom.common.js HTTP/1.1 Host: www.blogger.com User-Agent: Mozilla/5.0 (…) Gecko/2008070208 Firefox/3.0.1 Accept-Encoding: gzip,deflate HTTP/1.1 200 OK Content-Type: application/x-javascript Last-Modified: Mon, 22 Sep 2008 21:14:35 GMT Content-Length: 6230 function d(s) {... HTTP/1.1 200 OK Content-Type: application/x-javascript Last-Modified: Mon, 22 Sep 2008 21:14:35 GMT Content-Length: 2066 Content-Encoding: gzip XmoÛHþ\ÿFÖvã*wØoq...

  4. Gzip vs. Deflate gzip (default settings) compresses more

  5. Pros and Cons Pro: smaller transfer size Con: CPU cycles – on client and server Don't compress resources < 1K

  6. Gzip configuration Apache 1.3: mod_gzip mod_gzip_item_include file \.html$ mod_gzip_item_include mime ^text/html$ mod_gzip_item_include file \.js$ mod_gzip_item_include mime ^application/x-javascript$ mod_gzip_item_include file \.css$ mod_gzip_item_include mime ^text/css$ Apache 2.x: mod_deflate AddOutputFilterByType DEFLATE text/html text/css application/x-javascript control compression level: DeflateCompressionLevel http://httpd.apache.org/docs/2.0/mod/mod_deflate.html

  7. Gzip: not just for HTML March 2007 gzip scripts, stylesheets, XML, JSON (not images, Flash, PDF) October 2008

  8. Edge Case: Proxies Origin Server Proxy 2 GET main.js Accept-Encoding: gzip 1 GET main.js Accept-Encoding: gzip 5 main.js Content-Encoding: gzip 3 main.js Content-Encoding: gzip 6 GET main.js (no Accept-Encoding) 7main.js Content-Encoding: gzip 4 main.js Content-Encoding: gzip proxies may serve gzipped content to browsers that don't support it, and vice versa

  9. Edge Case: Proxies w/ Vary Origin Server Proxy 2 GET main.js Accept-Encoding: gzip 1 GET main.js Accept-Encoding: gzip 7 GET main.js (no Accept-Encoding) 5 main.js Content-Encoding: gzip 3 main.js Content-Encoding: gzip Vary: Accept-Encoding 6 GET main.js (no Accept-Encoding) 8 main.js Vary: Accept-Encoding 10 main.js (no gzip) 4 main.js Content-Encoding: gzip [Accept-Encoding: gzip] 11 GET main.js Accept-Encoding: gzip 9 main.js [Accept-Encoding: ] 12 main.js Content-Encoding: gzip 13 GET main.js (no Accept-Encoding) 14 main.js (no gzip) add Vary: Accept-Encoding

  10. Edge Case: Bad Browsers < 1% of browsers have problems with gzip IE 5.5: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q313712 IE 6.0: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q31249 Netscape 3.x, 4.x http://www.schroepl.net/projekte/mod_gzip/browser.htm User-Agent white list for gzip Apache 1.3: mod_gzip_item_include reqheader "User-Agent: MSIE [6-9]" mod_gzip_item_include reqheader "User-Agent: Mozilla/[5-9]" Apache 2.0: BrowserMatch ^MSIE [6-9] gzip BrowserMatch ^Mozilla/[5-9] gzip

  11. Edge Case: Bad Browsers (cont'd) proxies could mix-up responses give cached response from useragent1 to useragent2 could add Vary: User-Agent so many possibilities, defeats proxy caching better to add Cache-Control: Private downside: disables all proxy caches is it a serious problem? hard to diagnose; problem getting smaller

  12. Edge Case: ETags what happens when proxy makes Conditional GET requests? Last-Modified date for gzipped vs. ungzipped is different => If-Modified-Since works fine ETag is the same in Apache for gzipped & ungzipped => If-None-Match succeeds, proxy could give browser mismatched content remove Etags! (Rule 13) http://issues.apache.org/bugzilla/show_bug.cgi?id=39727

  13. Edge Case: ETags present Origin Server Proxy 2 GET main.js Accept-Encoding: gzip 1 GET main.js Accept-Encoding: gzip 7 GET main.js If-None-Match: "de158-e58-c7ee4140" 5 main.js Content-Encoding: gzip 3 main.js Content-Encoding: gzip Cache-Control: max-age=0 ETag: "de158-e58-c7ee4140" 6 GET main.js (no Accept-Encoding) 8 304 Not Modified 9 main.js Content-Encoding: gzip 4 main.js Content-Encoding: gzip Cache-Control: max-age=0 ETag: "de158-e58-c7ee4140" proxy gives browser mismatched content

  14. Edge Case: ETags removed Origin Server Proxy 2 GET main.js Accept-Encoding: gzip 1 GET main.js Accept-Encoding: gzip 7 GET main.js If-Modified-Since: Thu, 21 Aug 2008 23:53:57 GMT 5 main.js Content-Encoding: gzip 3 main.js Content-Encoding: gzip Cache-Control: max-age=0 Last-Modified: Thu, 21 Aug 2008 23:53:57 GMT 6 GET main.js (no Accept-Encoding) 8main.js Cache-Control: max-age=0 Last-Modified: Fri, 22 Aug 2008 09:43:15 GMT 10 main.js (no gzip) 4 main.js Content-Encoding: gzip Cache-Control: max-age=0 Last-Modified: Thu, 21 Aug 2008 23:53:57 GMT 9 main.js Cache-Control: max-age=0 Last-Modified: Fri, 22 Aug 2008 09:43:15 GMT removing ETags avoids the problem

  15. Edge Case Fixes March 2007 Vary: User-Agent – not used October 2008

  16. Homework "Improving Top Site" class project: add improvements for Rule 4 measure improvements using Hammerhead record results in your personal Web 100 sheet read Chapter 5 of HPWS for 10/17

  17. Questions How much are file sizes typically reduced by using gzip compression? What types of resources (images, scripts, etc.) should not be compressed? For the resource types that should be compressed, should they always be compressed? How do you prevent proxies from serving gzipped resources to browsers that don't support gzip? How can ETags cause proxies to serve mismatched content to browsers?

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