[Xcb] [Bug 29561] New: _XReply: performance regression in latest version

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=29561

           Summary: _XReply: performance regression in latest version
           Product: xorg
           Version: git
          Platform: Other
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: Lib/Xlib
        AssignedTo: xorg-team at lists.x.org
        ReportedBy: rami.ylimaki at vincit.fi
         QAContact: xorg-team at lists.x.org
                CC: xcb at lists.freedesktop.org


Compile the attached test program and run it with latest libX11 from FDO git.
Use "ltrace -S ./test-xreply" to monitor system calls.

The test program executes a single XInternAtoms request with 100 atoms. As a
result, _XReply is calling non-blocking read 200 times. It seems that whenever
_XReply reads n replies, it will execute n*2 non-blocking reads that all fail
with EAGAIN.

I tested the execution time of XInternAtoms with 100 atoms on a Debian Sid
snapshot from today. The libX11 from Sid doesn't suffer from this problem and
there is only one failing non-blocking read in _XReply. Then I compiled latest
upstream libX11 inside Sid and installed it. Updating libX11 made the problem
visible. On my machine, the XInternAtoms call with 100 atoms takes about 8
times longer (1ms vs. 8ms) if latest libX11 is used from FDO. I used ltrace for
getting the execution times.

I'm not too concerned about the performance of XInternAtoms itself, but it is
the easiest way to make this problem visible. The number of failing
non-blocking reads has increased for other requests as well, but there are
usually much less replies when handling other requests.

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