netdev - Re: Mainline kernel OLTP performance update

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Date:	Wed, 21 Jan 2009 18:58:10 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
cc:	Andi Kleen <andi@...stfloor.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	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 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.

The queues of the page allocator are of limited use due to their overhead.
Order-1 allocations can actually be 5% faster than order-0. order-0 makes
sense if pages are pushed rapidly to the page allocator and are then
reissues elsewhere. If there is a linear consumption then the page
allocator queues are just overhead.

> Page allocator has an array at zone_pcp(zone, cpu)->pcp to keep a page buffer for page order 0.
> But here skbuff_head_cache's order is 1, so UDP-U-4k couldn't benefit from the page buffer.

That usually does not matter because of partial list avoiding page
allocator actions.

> SLQB has no such issue, because:
> 1) SLQB has a percpu freelist. Free objects are put to the list firstly and can be picked up
> later on quickly without lock. A batch parameter to control the free object recollection is mostly
> 1024.
> 2) SLQB slab order mostly is 0, so although sometimes it calls alloc_pages/free_pages, it can
> benefit from zone_pcp(zone, cpu)->pcp page buffer.
>
> So SLUB need resolve such issues that one process allocates a batch of objects and another process
> frees them batchly.

SLUB has a percpu freelist but its bounded by the basic allocation unit.
You can increase that by modifying the allocation order. Writing a 3 or 5
into the order value in /sys/kernel/slab/xxx/order would do the trick.