netdev - Re: [PATCH v2 net-next] inet: Get critical word in first 64bit of cache line

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Date:	Sun, 03 Feb 2013 16:25:09 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	ling.ma.program@...il.com, netdev@...r.kernel.org, maze@...gle.com
Subject: Re: [PATCH v2 net-next] inet: Get critical word in first 64bit of
 cache line

On Sun, 2013-02-03 at 16:18 -0800, Eric Dumazet wrote:
> On Sun, 2013-02-03 at 16:08 -0500, David Miller wrote:
> > From: Eric Dumazet <eric.dumazet@...il.com>
> > Date: Sat, 02 Feb 2013 07:03:55 -0800
> > 
> > > From: Ma Ling <ling.ma.program@...il.com>
> > > 
> > > In order to reduce memory latency when last level cache miss occurs,
> > > modern CPUs i.e. x86 and arm introduced Critical Word First(CWF) or
> > > Early Restart(ER) to get data ASAP. For CWF if critical word is first
> > > member
> > > in cache line, memory feed CPU with critical word, then fill others
> > > data in cache line one by one, otherwise after critical word it must
> > > cost more cycle to fill the remaining cache line. For Early First CPU
> > > will restart until critical word in cache line reaches.
> > > 
> > > Hash value is critical word, so in this patch we place it as first
> > > member in cache line (sock address is cache-line aligned), and it is
> > > also good for Early Restart platform as well .
> > > 
> > > [edumazet: respin on net-next after commit ce43b03e8889]
> > > 
> > > Signed-off-by: Ma Ling <ling.ma.program@...il.com>
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > 
> > I completely agree with the other response to this patch in that
> > the description is bogus.
> > 
> > If CWF is implemented in the cpu, it should exactly relieve us from
> > having to move things around in structures so carefully like this.
> > 
> > Either the patch should be completely dropped (modern cpus don't
> > need this) or the commit message changed to reflect reality.
> > 
> > It really makes a terrible impression upon me when the patch says
> > something which in fact is 180 degrees from reality.
> 
> Hmm. 
> 
> Maybe the changelog is misleading, or maybe all the performance gains I
> have from this patch are probably some artifact or old/bad hardware, or
> something else.
> 
> 
> 
> (Intel(R) Xeon(R) CPU           X5660  @ 2.80GHz)
> # ./cwf
> looking-up aligned time 108712072, 
> looking-up unaligned time 113268256
> looking-up aligned time 108677032, 
> looking-up unaligned time 113297636
> 
> 
> (Intel(R) Xeon(R) CPU           X5679  @ 3.20GHz)
> # ./cwf
> looking-up aligned time 139193589, 
> looking-up unaligned time 144307821
> looking-up aligned time 139136787, 
> looking-up unaligned time 144277752
> 
> My laptop : Intel(R) Core(TM) i5-2520M CPU @ 2.50GHz
> # ./cwf
> looking-up aligned time 84869203, 
> looking-up unaligned time 86843462
> looking-up aligned time 84253003, 
> looking-up unaligned time 86227675
> 
> #include <stdio.h>
> #include <string.h>
> #include <stdlib.h>
> #include <unistd.h>
> 
> #define CACHELINE_SZ 64L
> 
> #define BIGBUFFER_SZ (64<<20)
> 
> # define HP_TIMING_NOW(Var) \
>  ({ unsigned long long _hi, _lo; \
>   asm volatile ("rdtsc" : "=a" (_lo), "=d" (_hi)); \
>   (Var) = _hi << 32 | _lo; })
> 
> #define repeat_times  20
> 
> char *bufzap;
> 
> static void zap_cache(void)
> {
> 	memset(bufzap, 2, BIGBUFFER_SZ);
> 	memset(bufzap, 3, BIGBUFFER_SZ);
> 	memset(bufzap, 4, BIGBUFFER_SZ);
> }
> 
> static char *init_buf(void)
> {
> 	void *res;
> 
> 	if (posix_memalign(&res, CACHELINE_SZ, BIGBUFFER_SZ)) {
> 		fprintf(stderr, "malloc() failed");
> 	        exit(1);
> 	}
> 
> 	memset(res, 1, BIGBUFFER_SZ);
> 	return res;
> }
> 
> unsigned long total;
> 
> static unsigned long random_access(void *buffer,
> 				   unsigned int off1,
> 				   unsigned int off2,
> 				   unsigned int off3)
> {
> 	int i;
> 	unsigned int n;
> 	unsigned long sum = 0;
> 	unsigned long *ptr;
> 
> 	srandom(7777);
> 	for (i = 0; i < 1000000; i++) {
> 		n = random() % (BIGBUFFER_SZ/CACHELINE_SZ);
> 		ptr = buffer + n*CACHELINE_SZ;
> 		if (ptr[off1])
> 			sum++;
> 		if (ptr[off2])
> 			sum++;
> //		if (ptr[off3])
> //			sum++;

Hmm, I don't know why I left a comment on these two lines...

Of course, results are a bit different removing the comments :

looking-up aligned time 113601316, 
looking-up unaligned time 115964760
looking-up aligned time 113698636, 
looking-up unaligned time 115986072

More testing is probably needed.


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