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SubjectRE: Kmem_cache handling in linux-2.6.2x kernel
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Hi;

>On Wed, Jul 9, 2008 at 8:06 AM, <KokHow.Teh@infineon.com> 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 my applications, only part of total number of objects of the
kmem_cache are freed. So my question is what happen to these "freed"
objects? How are these "freed" objects managed by the linux-MM? Would
they be reused by other kmem_cache_alloc() and/or kmalloc()?

>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.

(1) SLUB is not available in 2.6.20 kernel which I am using for my
products. In this case, is there similar mechamisms in place to
faciliate "page-sharing" amongst the kmem_cache_alloc() callers?

(2) Does this "page-sharing" happen for kmalloc() callers?

On Wed, Jul 9, 2008 at 8:06 AM, <KokHow.Teh@infineon.com> 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).

Thanks.

Regards,
KH


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