Re: Kmem_cache handling in linux-2.6.2x kernel

[Christoph Lameter <cl@linux-foundation.org>]

KokHow.Teh@infineon.com wrote:
> Hi list;
> 	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.
> 
> 	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()?
> 
> 	Any insight and advice is appreciated.

Any kmem_cache allocation and frees larger than PAGE_SIZE are converted to page allocator allocs and frees. Thus you are allocating and freeing directly from the general pool.

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