Re: [PATCH] x86: Run checksumming in parallel accross multiple alu's

[Joe Perches]

On Tue, 2013-10-15 at 09:41 +0200, Ingo Molnar wrote:
> * Joe Perches <joe@xxxxxxxxxxx> wrote:
> 
> > On Mon, 2013-10-14 at 15:44 -0700, Eric Dumazet wrote:
> > > On Mon, 2013-10-14 at 15:37 -0700, Joe Perches wrote:
> > > > On Mon, 2013-10-14 at 15:18 -0700, Eric Dumazet wrote:
> > > > > attached patch brings much better results
> > > > > 
> > > > > lpq83:~# ./netperf -H 7.7.8.84 -l 10 -Cc
> > > > > MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 7.7.8.84 () port 0 AF_INET
> > > > > Recv   Send    Send                          Utilization       Service Demand
> > > > > Socket Socket  Message  Elapsed              Send     Recv     Send    Recv
> > > > > Size   Size    Size     Time     Throughput  local    remote   local   remote
> > > > > bytes  bytes   bytes    secs.    10^6bits/s  % S      % S      us/KB   us/KB
> > > > > 
> > > > >  87380  16384  16384    10.00      8043.82   2.32     5.34     0.566   1.304  
> > > > > 
> > > > > diff --git a/arch/x86/lib/csum-partial_64.c b/arch/x86/lib/csum-partial_64.c
> > > > []
> > > > > @@ -68,7 +68,8 @@ static unsigned do_csum(const unsigned char *buff, unsigned len)
> > > > >  			zero = 0;
> > > > >  			count64 = count >> 3;
> > > > >  			while (count64) { 
> > > > > -				asm("addq 0*8(%[src]),%[res]\n\t"
> > > > > +				asm("prefetch 5*64(%[src])\n\t"
> > > > 
> > > > Might the prefetch size be too big here?
> > > 
> > > To be effective, you need to prefetch well ahead of time.
> > 
> > No doubt.
> 
> So why did you ask then?
> 
> > > 5*64 seems common practice (check arch/x86/lib/copy_page_64.S)
> > 
> > 5 cachelines for some processors seems like a lot.
> 
> What processors would that be?

The ones where conservatism in L1 cache use is good
because there are multiple threads running concurrently.

> Most processors have hundreds of cachelines even in their L1 cache. 

And sometimes that many executable processes too.

> Thousands in the L2 cache, up to hundreds of thousands.

Irrelevant because prefetch doesn't apply there.

Ingo, Eric _showed_ that the prefetch is good here.
How about looking at a little optimization to the minimal
prefetch that gives that level of performance.

You could argue that prefetching PAGE_SIZE or larger
would be better still otherwise.

I suspect that using a smaller multiple of
L1_CACHE_BYTES like 2 or 3 would perform the same.

The last time it was looked at for copy_page_64.S was
quite awhile ago.  It looks like maybe 2003.


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