llvm dev - Aug 2014 - [LLVMdev] Recent compile time performance regressions

On 01/08/2014 22:07, Chandler Carruth wrote:
> Note that I've fixed one bad compile time regression quite recently,
and
> we're bisecting to another one. We benchmarked the multithreading stuff
> pretty carefully, so I doubt its that. Have you tried reverting locally and
> reproducing?
Not really. It just saw this passing by and was wondering if it raised 
some bells.

The reason I pointed this patch out is that the pattern in which 
performance changes matches precisely the pattern this patch was applied 
and reverted, with the set of patches between subsequent performance 
runs being rather small.

Note, this patch is actually the smaller culprit, around 209801 there is 
another performance regression, but I have no idea if/where this is 
coming from or if this for some reason is a false positive.

I unfortunately currently don't have the time to investigate this, but 
thought I at least mention it on the mailing list.

Cheers,
Tobias

P.S: Until the last performance build at juli 31st the performance 
regression still existed, so I don't think your fix applied to this one.

Not asking you to do anything specifically Tobias, just adding information
to the list...

On Fri, Aug 1, 2014 at 1:16 PM, Tobias Grosser <tobias at grosser.es>
wrote:
> Note, this patch is actually the smaller culprit, around 209801 there is
> another performance regression, but I have no idea if/where this is coming
> from or if this for some reason is a false positive.

I'm having to question the results on this bot. The range of commits which
cause the larger compile time regression is 209797 - 209799... But those
commits are totally innocuous. The closest to a something that could go
poorly would be r209798, but that commit doesn't seem plausible for a huge
slowdown.

I wonder if there is something weird going on with the bot...
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On 1 August 2014 21:33, Chandler Carruth <chandlerc at google.com>
wrote:
>> Note, this patch is actually the smaller culprit, around 209801 there
is
>> another performance regression, but I have no idea if/where this is
coming
>> from or if this for some reason is a false positive.
>
>
> I'm having to question the results on this bot. The range of commits
which
> cause the larger compile time regression is 209797 - 209799... But those
> commits are totally innocuous. The closest to a something that could go
> poorly would be r209798, but that commit doesn't seem plausible for a
huge
> slowdown.
>
> I wonder if there is something weird going on with the bot...
209797-209799 regression is false positive, due to reverting perf back
to original timeit tool.(r209797)

However the multithreading stuff is real regression and I'm able to
reproduce same regression on my local build bots.

-Yi