netdev - Re: Mainline kernel OLTP performance update

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Date:	Thu, 22 Jan 2009 11:15:07 +0200
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Cc:	Christoph Lameter <cl@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Andrew Morton <akpm@...ux-foundation.org>,
	netdev@...r.kernel.org, sfr@...b.auug.org.au,
	matthew.r.wilcox@...el.com, chinang.ma@...el.com,
	linux-kernel@...r.kernel.org, sharad.c.tripathi@...el.com,
	arjan@...ux.intel.com, suresh.b.siddha@...el.com,
	harita.chilukuri@...el.com, douglas.w.styner@...el.com,
	peter.xihong.wang@...el.com, hubert.nueckel@...el.com,
	chris.mason@...cle.com, srostedt@...hat.com,
	linux-scsi@...r.kernel.org, andrew.vasquez@...gic.com,
	anirban.chakraborty@...gic.com
Subject: Re: Mainline kernel OLTP performance update

On Thu, 2009-01-22 at 16:36 +0800, Zhang, Yanmin wrote:
> On Wed, 2009-01-21 at 18:58 -0500, Christoph Lameter wrote:
> > On Tue, 20 Jan 2009, Zhang, Yanmin wrote:
> > 
> > > kmem_cache skbuff_head_cache's object size is just 256, so it shares the kmem_cache
> > > with :0000256. Their order is 1 which means every slab consists of 2 physical pages.
> > 
> > That order can be changed. Try specifying slub_max_order=0 on the kernel
> > command line to force an order 0 alloc.
> I tried slub_max_order=0 and there is no improvement on this UDP-U-4k issue.
> Both get_page_from_freelist and __free_pages_ok's cpu time are still very high.
> 
> I checked my instrumentation in kernel and found it's caused by large object allocation/free
> whose size is more than PAGE_SIZE. Here its order is 1.
> 
> The right free callchain is __kfree_skb => skb_release_all => skb_release_data.
> 
> So this case isn't the issue that batch of allocation/free might erase partial page
> functionality.

So is this the kfree(skb->head) in skb_release_data() or the put_page()
calls in the same function in a loop?

If it's the former, with big enough size passed to __alloc_skb(), the
networking code might be taking a hit from the SLUB page allocator
pass-through.

		Pekka

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