Re: [Qemu-devel] [RFC PATCH] qht: Align sequence lock to cache line

[Pranith Kumar]

On Tue, Oct 25, 2016 at 11:41 AM, Paolo Bonzini <address@hidden> 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