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SubjectRe: [RFC] net: further seperate dst_entry.__refcnt from cache contention
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On Fri, 2012-07-20 at 14:46 -0500, Nathan Zimmer wrote:
> After some investigation on large machines I found that
> dst_entry.__refcnt particpates in false cache sharing issues that show
> when scaling past 12 threads who communicate via tcp with loopback addresses.
> I adjusted refcnt to be on its own cache line and that helped quite a bit.
> But perhaps a bit of a waste of space? Is there some better way?
>
> Here is some preliminary data I had gathered. It shows nicely improved scaling.
>
> Threads baseline afterchange
> 2 1328.03 1340.67
> 4 2430.31 2282.09
> 6 3087.65 3258.12
> 8 3560.34 4165.88
> 10 3900.34 4962.28
> 12 3933.38 5613.76
> 14 3876.98 6113.85
> 16 3706.01 6338.00
> 18 3742.48 6634.77
> 20 3670.15 6641.25
> 22 3660.98 6799.55
> 24 3503.13 6613.45
> 26 3525.73 6702.67
> 28 3440.16 6801.27
> 30 3497.59 6911.52
> 32 3498.25 6540.06
>
> I should say something about this test. It is a dead simple test in which a
> pair of threads simply pass data to each other. They were placed in the same
> socket to avoid cross node overhead.
>
> CC: "David S. Miller" <davem@davemloft.net>
> Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
>
> ---
> include/net/dst.h | 2 +-
> 1 files changed, 1 insertions(+), 1 deletions(-)
>
> diff --git a/include/net/dst.h b/include/net/dst.h
> index 8197ead..3898643 100644
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -84,7 +84,7 @@ struct dst_entry {
> * input/output/ops or performance tanks badly
> */
> atomic_t __refcnt; /* client references */
> - int __use;
> + int __use ____cacheline_aligned;
> unsigned long lastuse;
> union {
> struct dst_entry *next;

Its a known problem, and we are waiting IP cache removal to address it.

Before the cache removal, a machine can have million of dst

Another idea concerning very hot dst would be to clone them on demand.





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