Re: Mainline kernel OLTP performance update

From: Christoph Lameter
Date: Wed Jan 21 2009 - 19:31:50 EST

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.

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.