Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

[James Bottomley]

On Wed, 2011-05-11 at 13:38 -0700, David Rientjes wrote:
> On Wed, 11 May 2011, Mel Gorman wrote:
> 
> > To avoid locking and per-cpu overhead, SLUB optimisically uses
> > high-order allocations up to order-3 by default and falls back to
> > lower allocations if they fail. While care is taken that the caller
> > and kswapd take no unusual steps in response to this, there are
> > further consequences like shrinkers who have to free more objects to
> > release any memory. There is anecdotal evidence that significant time
> > is being spent looping in shrinkers with insufficient progress being
> > made (https://lkml.org/lkml/2011/4/28/361) and keeping kswapd awake.
> > 
> > SLUB is now the default allocator and some bug reports have been
> > pinned down to SLUB using high orders during operations like
> > copying large amounts of data. SLUBs use of high-orders benefits
> > applications that are sized to memory appropriately but this does not
> > necessarily apply to large file servers or desktops.  This patch
> > causes SLUB to use order-0 pages like SLAB does by default.
> > There is further evidence that this keeps kswapd's usage lower
> > (https://lkml.org/lkml/2011/5/10/383).
> > 
> 
> This is going to severely impact slub's performance for applications on 
> machines with plenty of memory available where fragmentation isn't a 
> concern when allocating from caches with large object sizes (even 
> changing the min order of kamlloc-256 from 1 to 0!) by default for users 
> who don't use slub_max_order=3 on the command line.  SLUB relies heavily 
> on allocating from the cpu slab and freeing to the cpu slab to avoid the 
> slowpaths, so higher order slabs are important for its performance.
> 
> I can get numbers for a simple netperf TCP_RR benchmark with this change 
> applied to show the degradation on a server with >32GB of RAM with this 
> patch applied.
> 
> It would be ideal if this default could be adjusted based on the amount of 
> memory available in the smallest node to determine whether we're concerned 
> about making higher order allocations.  (Using the smallest node as a 
> metric so that mempolicies and cpusets don't get unfairly biased against.)  
> With the previous changes in this patchset, specifically avoiding waking 
> kswapd and doing compaction for the higher order allocs before falling 
> back to the min order, it shouldn't be devastating to try an order-3 alloc 
> that will fail quickly.

So my testing has shown that simply booting the kernel with
slub_max_order=0 makes the hang I'm seeing go away.  This definitely
implicates the higher order allocations in the kswapd problem.  I think
it would be wise not to make it the default until we can sort out the
root cause.

James




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