Re: Mainline kernel OLTP performance update

[Zhang, Yanmin]

On Wed, 2009-01-21 at 18:58 -0500, Christoph Lameter wrote:
> On Tue, 20 Jan 2009, Zhang, Yanmin wrote:
> 
> > kmem_cache ïskbuff_head_cache's object size is just 256, so it shares the kmem_cache
> > with ï: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 ïslub_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.

'#slaninfo -AD' couldn't show statistics of large object allocation/free. Can we add
such info? That will be more helpful.

In addition, I didn't find such issue wih TCP stream testing.

> 
> 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 ïskbuff_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 ïzone_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.

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