Re: [patch] slub: default min_partial to at least highest cpus per node

[Pekka Enberg <penberg@cs.helsinki.fi>]

David Rientjes wrote:
>> The pre-defined MIN_PARTIAL value may not be suitable for machines with a
>> large number of cpus per node.  To avoid excessively allocating new slabs
>> because there is not at least the same number of slabs on a node's
>> partial list as cpus, this will default a cache's min_partial value to be
>> at least the highest number of cpus per node on the system.
>>
>> This default will never exceed MAX_PARTIAL, however, so very large
>> systems don't waste an excessive amount of memory.
>>
>> When remote_node_defrag_ratio allows defragmenting remote nodes, it
>> ensures that nr_partial exceeds min_partial so there will always be a
>> local reserve when a cpu slab is filled to avoid allocating new slabs
>> locally as a result of a remote cpu stealing a partial slab.
>>
>> The cache's min_partial setting may still be changed by writing to
>> /sys/kernel/slab/cache/min_partial.  The only restriction when doing so
>> is that the value be within MIN_PARTIAL and MAX_PARTIAL.
>>
>> Cc: Christoph Lameter <cl@linux-foundation.org>
>> Signed-off-by: David Rientjes <rientjes@google.com>

On Tue, Apr 7, 2009 at 9:58 PM, Pekka Enberg <penberg@cs.helsinki.fi> wrote:
> Hmm, partial lists are per-node, so wouldn't it be better to do the
> adjustment for every struct kmem_cache_node separately? The
> 'min_partial_per_node' global seems just too ugly and confusing to live
> with.

Btw, that requires moving ->min_partial to struct kmem_cache_node from
struct kmem_cache.  But I think that makes a whole lot of sense if
some nodes may have more CPUs than others.

And while the improvement is kinda obvious, I would be interested to
know what kind of workload benefits from this patch (and see numbers
if there are any).

                                  Pekka