Bug 102260 - 15% perf drop in DrvRes with "i965/miptree: Use num_samples of 1 instead of 0 for single-sampled"
Summary: 15% perf drop in DrvRes with "i965/miptree: Use num_samples of 1 instead of 0...
Status: VERIFIED FIXED
Alias: None
Product: Mesa
Classification: Unclassified
Component: Drivers/DRI/i965 (show other bugs)
Version: git
Hardware: Other All
: medium normal
Assignee: Intel 3D Bugs Mailing List
QA Contact: Intel 3D Bugs Mailing List
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Reported: 2017-08-16 13:37 UTC by Eero Tamminen
Modified: 2017-08-21 07:52 UTC (History)
0 users

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Attachments

Description Eero Tamminen 2017-08-16 13:37:28 UTC 
Bisecting on SKL GT2 revealed following commit:
----------------------------------------------------
commit 76e2f390f9863a356d1419982dec705260d67eff
Author:     Topi Pohjolainen <topi.pohjolainen@intel.com>
AuthorDate: Wed Jul 19 09:25:19 2017 +0300
Commit:     Topi Pohjolainen <topi.pohjolainen@intel.com>
CommitDate: Thu Jul 20 11:32:21 2017 +0300

    i965/miptree: Use num_samples of 1 instead of 0 for single-sampled
----------------------------------------------------

To be responsible for large performance drop in SynMark v7 DrvRes test.

Drop is largest (~15%) on SKL & KBL GT2, on most platforms it's 3-5%, on SKL GT4e there's no measurable impact.

(Test is run in FullHD fullscreen.)

From the commit message it seems that it should be just refactoring + potential bug fix i.e. this large perf impact is unexpected.  Other tests beside DrvRes weren't noticeably impacted.
Comment 1 Jason Ekstrand 2017-08-19 00:41:46 UTC 
I've verified that the stated commit does, indeed, cause the regression though I have no idea why yet.  It should have been an entirely innocuous change.
Comment 2 Jason Ekstrand 2017-08-19 18:09:04 UTC 
Patch on the list:

https://patchwork.freedesktop.org/patch/172628/
Comment 3 Jason Ekstrand 2017-08-19 22:40:48 UTC 
This is fixed by the following commit:

commit f24cf82d6db290a88abfff0669d2c5e2aa463901
Author: Jason Ekstrand <jason.ekstrand@intel.com>
Date:   Sat Aug 19 11:03:38 2017 -0700

    i965/tex: Don't pass samples to miptree_create_for_teximage
    
    In 76e2f390f9863a35, when Topi switched num_samples from 0 to 1 for
    single-sampled, he accidentally switched the last parameter in the call
    to miptree_create_for_teximage from 0 to 1 thinking it was num_samples
    when it was actually layout_flags.  Switching from 0 to 1 added the
    MIPTREE_LAYOUT_ACCELERATED_UPLOAD flag which causes us to allocate a
    busy BO instead of an idle one.  This caused the subsequent CPU upload
    to consistently stall.  The end result was a 15% performance drop in the
    SynMark v7 DrvRes microbenchmark.  This restores the old behavior and
    fixes the performance regression.
    
    Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
    Fixes: 76e2f390f9863a356d1419982dec705260d67eff
    Bugzilla: https://bugs.freedesktop.org/102260
    Cc: mesa-stable@lists.freedesktop.org
Comment 4 Eero Tamminen 2017-08-21 07:52:20 UTC 
Verified. DrvRes perf is now same, and in some cases slightly better, than it was before the drop.

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