Re: Mainline kernel OLTP performance update

Pekka Enberg
2009-01-22T09:15:10Z

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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