On Tue, Oct 25, 2016 at 11:41 AM, Paolo Bonzini <pbonz...@redhat.com> wrote: > > > On 25/10/2016 17:35, Pranith Kumar wrote: >> Using perf, I see that sequence lock is being a bottleneck since it is >> being read by everyone. Giving it its own cache-line seems to help >> things quite a bit. >> >> Using qht-bench, I measured the following for: >> >> $ ./tests/qht-bench -d 10 -n 24 -u <x> >> >> throughput base patch %change >> update >> 0 8.07 13.33 +65% >> 10 7.10 8.90 +25% >> 20 6.34 7.02 +10% >> 30 5.48 6.11 +9.6% >> 40 4.90 5.46 +11.42% >> >> I am not able to see any significant increases for lower thread counts >> though. > > Whoa, that's a bit of a heavy hammer. The idea is that whoever modifies > the seqlock must take the spinlock first, and whoever reads the seqlock > will read one of the members of hashes[]/pointers[]. So having > everything in the same cacheline should be better.
But we are taking the seqlock of only the head bucket, while the readers are reading hashes/pointers of the chained buckets. > > What happens if you just change QHT_BUCKET_ALIGN to 128? > 0 4.47 10 9.82 (weird!) 20 8.13 30 7.13 40 6.45 Thanks, -- Pranith
