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

[Kostya Serebryany]

On Fri, Apr 18, 2014 at 5:24 AM, Joerg Sonnenberger <joerg at britannica.bec.de
> wrote:

> On Thu, Apr 17, 2014 at 04:21:46PM +0400, Kostya Serebryany wrote:
> > - sharded counters: each counter represented as N counters sitting in
> > different cache lines. Every thread accesses the counter with index
> TID%N.
> > Solves the problem partially, better with larger values of N, but then
> > again it costs RAM.
>
> I'd strongly go with this schema with one tweak. Use the stack pointer
> as base value with some fudging to not just use the lowest bits. It is
> typically easier to get.
>

Indeed, several middle bits of %sp may be used instead of TID%N.
This would heavily depend on the pthread implementation (how it allocates
stacks) though.
It may be tricky to come up with the same constant scheme across all
platforms.


>
> Joerg
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