Messages in this thread | | | Subject | RE: Kmem_cache handling in linux-2.6.2x kernel | Date | Wed, 9 Jul 2008 18:30:26 +0800 | From | <> |
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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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