[LLVMdev] multithreaded performance disaster with -fprofile-instr-generate (contention on profile counters)

[Bob Wilson]

On Apr 17, 2014, at 11:09 AM, Xinliang David Li <xinliangli at gmail.com> wrote:
> 
> On Thu, Apr 17, 2014 at 10:58 AM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
> 
> On 2014-Apr-17, at 10:38, Xinliang David Li <xinliangli at gmail.com> wrote:
> 
> >
> > Another idea is to use stack local counters per function -- synced up with global counters on entry and exit. the problem with it is for deeply recursive calls, stack pressure can be too high.
> 
> I think they'd need to be synced with global counters before function
> calls as well, since any function call can call "exit()".
> 
> right -- but it might be better to handle this in other ways. For instance a stack of counters for each frames is maintained. At exit, they are flushed in a batch. Or simply ignore it in case of program exit .

It seems to me like we’re going to have a hard time getting good multithreaded performance without significant impact on the single-threaded behavior. We might need to add an option to choose between those. There’s a lot of room for improvement in the performance with the current instrumentation, so maybe we can find a way to make things incrementally better in a way that helps both, but avoiding the multithreaded cache conflicts seems like it’s going to be expensive in other ways.
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