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net: reorder struct Qdisc for better SMP performance

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dev_queue_xmit() needs to dirty fields "state", "q", "bstats" and "qstats"

On x86_64 arch, they currently span three cache lines, involving more
cache line ping pongs than necessary, making longer holding of queue spinlock.

We can reduce this to one cache line, by grouping all read-mostly fields
at the beginning of structure. (Or should I say, all highly modified fields
at the end :) )

Before patch :

offsetof(struct Qdisc, state)=0x38
offsetof(struct Qdisc, q)=0x48
offsetof(struct Qdisc, bstats)=0x80
offsetof(struct Qdisc, qstats)=0x90
sizeof(struct Qdisc)=0xc8

After patch :

offsetof(struct Qdisc, state)=0x80
offsetof(struct Qdisc, q)=0x88
offsetof(struct Qdisc, bstats)=0xa0
offsetof(struct Qdisc, qstats)=0xac
sizeof(struct Qdisc)=0xc0

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet
2009-03-20 01:33:32 -07:00
committed by David S. Miller
parent 408896d508
commit 5e140dfc1f
2 changed files with 13 additions and 10 deletions
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2
include/linux/gen_stats.h
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@@ -22,7 +22,7 @@ struct gnet_stats_basic
{
__u64 bytes;
__u32 packets;
};
} __attribute__ ((packed));
/**
* struct gnet_stats_rate_est - rate estimator