Re: Mainline kernel OLTP performance update

["Zhang, Yanmin" <yanmin_zhang@linux.intel.com>]

On Thu, 2009-01-22 at 11:47 +0200, Pekka Enberg wrote:
> On Thu, 2009-01-22 at 17:28 +0800, Zhang, Yanmin wrote:
> > On Thu, 2009-01-22 at 11:15 +0200, Pekka Enberg wrote:
> > > 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?
> > It's kfree(skb->head).
> > 
> > > 
> > > 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.
> 
> Do we know what kind of size is being passed to __alloc_skb() in this
> case?
In function __alloc_skb, original parameter size=4155,
SKB_DATA_ALIGN(size)=4224, sizeof(struct skb_shared_info)=472, so
__kmalloc_track_caller's parameter size=4696.

>  Maybe we want to do something like this.
> 
> 		Pekka
> 
> SLUB: revert page allocator pass-through
This patch amost fixes the netperf UDP-U-4k issue.

#slabinfo -AD
Name                   Objects    Alloc     Free   %Fast
:0000256                  1658 70350463 70348946  99  99 
kmalloc-8192                31 70322309 70322293  99  99 
:0000168                  2592   143154   140684  93  28 
:0004096                  1456    91072    89644  99  96 
:0000192                  3402    63838    60491  89  11 
:0000064                  6177    49635    43743  98  77 

So kmalloc-8192 appears. Without the patch, kmalloc-8192 hides.
kmalloc-8192's default order on my 8-core stoakley is 2.

1) If I start CPU_NUM clients and servers, SLUB's result is about 2% better than SLQB's;
2) If I start 1 clinet and 1 server, and bind them to different physical cpu, SLQB's result
is about 10% better than SLUB's.

I don't know why there is still 10% difference with item 2). Maybe cachemiss causes it?

> 
> This is a revert of commit aadb4bc4a1f9108c1d0fbd121827c936c2ed4217 ("SLUB:
> direct pass through of page size or higher kmalloc requests").
> ---
> 
> diff --git a/include/linux/slub_def.h b/include/linux/slub_def.h
> index 2f5c16b..3bd3662 100644
> --- a/include/linux/slub_def.h
> +++ b/include/linux/slub_def.h