Commit f1dd9c37 authored Mar 12, 2008 by Zhang Yanmin Committed by David S. Miller Mar 12, 2008

[NET]: Fix tbench regression in 2.6.25-rc1



Comparing with kernel 2.6.24, tbench result has regression with
2.6.25-rc1.

1) On 2 quad-core processor stoakley: 4%.
2) On 4 quad-core processor tigerton: more than 30%.

bisect located below patch.

b4ce9277 is first bad commit
commit b4ce9277
Author: Herbert Xu <herbert@gondor.apana.org.au>
Date:   Tue Nov 13 21:33:32 2007 -0800

    [IPV6]: Move nfheader_len into rt6_info

    The dst member nfheader_len is only used by IPv6.  It's also currently
    creating a rather ugly alignment hole in struct dst.  Therefore this patch
    moves it from there into struct rt6_info.

Above patch changes the cache line alignment, especially member
__refcnt. I did a testing by adding 2 unsigned long pading before
lastuse, so the 3 members, lastuse/__refcnt/__use, are moved to next
cache line. The performance is recovered.

I created a patch to rearrange the members in struct dst_entry.

With Eric and Valdis Kletnieks's suggestion, I made finer arrangement.

1) Move tclassid under ops in case CONFIG_NET_CLS_ROUTE=y. So
   sizeof(dst_entry)=200 no matter if CONFIG_NET_CLS_ROUTE=y/n. I
   tested many patches on my 16-core tigerton by moving tclassid to
   different place. It looks like tclassid could also have impact on
   performance.  If moving tclassid before metrics, or just don't move
   tclassid, the performance isn't good. So I move it behind metrics.

2) Add comments before __refcnt.

On 16-core tigerton:

If CONFIG_NET_CLS_ROUTE=y, the result with below patch is about 18%
better than the one without the patch;

If CONFIG_NET_CLS_ROUTE=n, the result with below patch is about 30%
better than the one without the patch.

With 32bit 2.6.25-rc1 on 8-core stoakley, the new patch doesn't
introduce regression.

Thank Eric, Valdis, and David!

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Acked-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 22626216

include/net/dst.h

+14 −9

Original line number	Diff line number	Diff line
		@@ -52,15 +52,10 @@ struct dst_entry
		unsigned short header_len; /* more space at head required */
		unsigned short trailer_len; /* space to reserve at tail */

		u32 metrics[RTAX_MAX];
		struct dst_entry *path;

		unsigned long rate_last; /* rate limiting for ICMP */
		unsigned int rate_tokens;
		unsigned long rate_last; /* rate limiting for ICMP */

		#ifdef CONFIG_NET_CLS_ROUTE
		__u32 tclassid;
		#endif
		struct dst_entry *path;

		struct neighbour *neighbour;
		struct hh_cache *hh;
		@@ -71,9 +66,19 @@ struct dst_entry

		struct dst_ops *ops;

		unsigned long lastuse;
		u32 metrics[RTAX_MAX];

		#ifdef CONFIG_NET_CLS_ROUTE
		__u32 tclassid;
		#endif

		/*
		* __refcnt wants to be on a different cache line from
		* input/output/ops or performance tanks badly
		*/
		atomic_t __refcnt; /* client references */
		int __use;
		unsigned long lastuse;
		union {
		struct dst_entry *next;
		struct rtable *rt_next;

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