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Date: Fri, 14 Nov 2008 16:22:30 +0300 From: Alexey Dobriyan <adobriyan@...il.com> To: Eric Dumazet <dada1@...mosbay.com> Cc: David Miller <davem@...emloft.net>, netdev@...r.kernel.org, shemminger@...tta.com, "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> Subject: Re: [PATCH] net: make sure struct dst_entry refcount is aligned on 64 bytes On Fri, Nov 14, 2008 at 12:43:06PM +0100, Eric Dumazet wrote: > Alexey Dobriyan a écrit : >> On Fri, Nov 14, 2008 at 11:47:01AM +0100, Eric Dumazet wrote: >>> Alexey Dobriyan a écrit : >>>> On Fri, Nov 14, 2008 at 10:04:24AM +0100, Eric Dumazet wrote: >>>>> David Miller a écrit : >>>>>> From: Eric Dumazet <dada1@...mosbay.com> >>>>>> Date: Fri, 14 Nov 2008 09:09:31 +0100 >>>>>> >>>>>>> During tbench/oprofile sessions, I found that dst_release() was in third position. >>>>>> ... >>>>>>> Instead of first checking the refcount value, then decrement it, >>>>>>> we use atomic_dec_return() to help CPU to make the right memory transaction >>>>>>> (ie getting the cache line in exclusive mode) >>>>>> ... >>>>>>> Signed-off-by: Eric Dumazet <dada1@...mosbay.com> >>>>>> This looks great, applied, thanks Eric. >>>>>> >>>>> Thanks David >>>>> >>>>> >>>>> I think I understood some regressions here on 32bits >>>>> >>>>> offsetof(struct dst_entry, __refcnt) is 0x7c again !!! >>>>> >>>>> This is really really bad for performance >>>>> >>>>> I believe this comes from a patch from Alexey Dobriyan >>>>> (commit def8b4faff5ca349beafbbfeb2c51f3602a6ef3a >>>>> net: reduce structures when XFRM=n) >>>> Ick. >>> Well, your patch is a good thing, we only need to make adjustments. >>> >>>>> This kills effort from Zhang Yanmin (and me...) >>>>> >>>>> (commit f1dd9c379cac7d5a76259e7dffcd5f8edc697d17 >>>>> [NET]: Fix tbench regression in 2.6.25-rc1) >>>>> >>>>> >>>>> Really we must find something so that this damned __refcnt is starting at 0x80 >>>> Make it last member? >>> Yes, it will help tbench, but not machines that stress IP route cache >>> >>> (dst_use() must dirty the three fields "refcnt, __use , lastuse" ) >>> >>> Also, 'next' pointer should be in the same cache line, to speedup route >>> cache lookups. >> >> Knowledge taken. >> >>> Next problem is that offsets depend on architecture being 32 or 64 bits. >>> >>> On 64bit, offsetof(struct dst_entry, __refcnt) is 0xb0 : not very good... >> >> I think all these constraints can be satisfied with clever rearranging of dst_entry. >> Let me come up with alternative patch which still reduces dst slab size. > > You cannot reduce size, and it doesnt matter, since we use dst_entry inside rtable > and rtable is using SLAB_HWCACHE_ALIGN kmem_cachep : we have many bytes available. > > After patch on 32 bits > > sizeof(struct rtable)=244 (12 bytes left) > > Same for other containers. Hmm, indeed. I tried moving __refcnt et al to the very beginning, but it seems to make things worse (on x86_64, almost within statistical error). And there is no way to use offset_of() inside struct definition. :-( -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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