Re: Mainline kernel OLTP performance update

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

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.