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https://issues.apache.org/jira/browse/TS-2368?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13825890#comment-13825890
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Mark Harrison commented on TS-2368:
-----------------------------------
It is.
I'm being stupid and was looking at things at a much too low level.
> read while writer functionality not working
> -------------------------------------------
>
> Key: TS-2368
> URL: https://issues.apache.org/jira/browse/TS-2368
> Project: Traffic Server
> Issue Type: Bug
> Components: Cache
> Reporter: Mark Harrison
>
> I'm seeing some unusual behavior when trying to use the read_while_writer
> functionality in ATS 4.0.2.
> My test case is a simple HTTP server that on each request will sleep for 5
> seconds and then return a response (simulating a slow/overloaded origin
> server). My test is to run curl against a URL and immediately (approx 1
> second later) run the same curl in another window.
> Curl command: curl -v -x 127.0.0.1:80 http://foo.com/5
> (a remap entry is in place for foo.com to the origin server)
> The response is cacheable - I set Cache-Control: max-age headers and see cS
> in the via header (or cH when there's a cache hit on subsequent requests).
> I have proxy.config.cache.enable_read_while_writer set to 1 in records.config.
> The expected behavior is that the second request will hang until the first
> request is complete, and both with return at the same time with the data, and
> that the origin server will see only one request.
> The observed behavior is that both requests take 5 seconds from starting
> them, and the origin server sees two requests.
> After much hunting around running ATS under gdb, I spotted where I think the
> issue is (although not why):
> iocore/cache/CacheRead.cc line 205: if (!w->closed &&
> !cache_config_read_while_writer)
> ATS thinks that cache_config_read_while_writer is 0 at this point, despite it
> being set in the config file. I discovered this by setting a breakpoint on
> CacheVC::openReadChooseWriter and stepping through. This behavior also
> happens if I compile with -O0.
> If I change the code to ignore the value of cache_config_read_while_writer
> (i.e. comment out that if statement), then I get the expected behavior and
> read_while_writer appears to work.
> My test machine is a vagrant vm, running centos 6.3, and ATS was compiled
> with ./configure CXXFLAGS=-O0 CFLAGS=-O0 --enable-layout=Gentoo
> --enable-experimental-plugins --libdir=/usr/lib64/trafficserver
> --with-tcl=/usr/lib64 --with-user=ats --with-group=ats
> (I also compiled without the CXXFLAGS and CFLAGS - I added those in to make
> working with gdb easier, but the behavior was the same).
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