Re: Kmem_cache handling in linux-2.6.2x kernel

[Pekka Enberg]

Hi,

On Wed, Jul 9, 2008 at 8:06 AM,  <KokHow.Teh@xxxxxxxxxxxx> wrote:
>        I have a question about kmem_cache implemented in Linux-2.6.2x
> kernel. I have an application that allocates and free 64KByte chunks of
> memory (32-byte aligned) quite often. Therefore, I create a lookaside
> cache for that purpose and use kmem_cache_alloc(), kmem_cache_free() to
> allocate and free the caches. The application works very well in this
> model. However, my concern here is if kmem_cache_free() does return the
> cache to the system-wide pool so that it could be used by other
> applications when need arises; when system is low in memory resources,
> for instance. This is a question about the internal workings of the
> memory management system of the Linux-2.6.2x kernel as to how efficient
> it manages this lookasie caches. The concern is valid because if this
> lookaside cache is not managed well, i.e, it is not returned to the
> system-wide free memory pools to be used by other applications, this
> will penalize the performace and throughput of the whole system due to
> the dynamic behaviour of the utilization of system memory resources. For
> example, other applications might be swapping in and out of the harddisk
> and if the kmem_cache_free()'ed memory objects could be used by these
> applications, it will help in this case to reduce the number of swaps
> that happen, thereby freeing the CPU and/or DMA from doing the swapping
> to do other critical tasks.

I'm not sure I understand the question. The pages allocated for a
particular cache are given back to the page allocator whenever all
objects of a slab are freed. In addition, SLUB does slab merging so
the same cache can be transparently used by other kmem_cache_alloc()
callers. So there really are no reservation guarantees for a cache in
OOM conditions.

On Wed, Jul 9, 2008 at 8:06 AM,  <KokHow.Teh@xxxxxxxxxxxx> wrote:
>        On the other hand, if the caches are returned to the system-wide
> free memory pool, what are the advantages of using kmem_cache_t compared
> to the conventional kmalloc()/kfree()?

The main advantages for using kmem_cache_create() are that (1) you
control the alignment and (2) you get tighter packing for the objects
(less internal fragmentation).
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