Re: The VFS cache is not freed when there is not enough free memory to allocate

Nick Piggin
2006-11-30T21:18:13Z

Aubrey wrote:
> On 11/29/06, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
>> That was the order-9 allocation failure. Which is not going to be
>> solved properly by just dropping caches.
>>
>> But Sonic apparently saw failures with 4K allocations, where the
>> caches weren't getting shrunk properly. This would be more interesting
>> because it would indicate a real problem with the kernel.
>>
> I have done several test cases. when cat /proc/meminfo show MemFree <
> 8192KB,
>
> 1) malloc(1024 * 4), 256 times = 8MB, allocation successful.
> 2) malloc(1024 * 16), 64 times = 8MB, allocation successful.
> 3) malloc(1024 * 64), 16 times = 8MB, allocation successful.
> 4) malloc(1024 * 128), 8 times = 8MB, allocation failed.
> 5) malloc(1024 * 256), 4 times = 8MB, allocation failed.
>
>> From those results, we know, when allocation <=64K, cache can be
>
> shrunk properly.
> That means the malloc size of an application on nommu should be
> <=64KB. That's exactly our problem. Some video programmes need a big
> block which has contiguous physical address. But yes, as you said, we
> must keep malloc not to alloc a big block to make the current kernel
> working robust on nommu.
>
> So, my question is, Can we improve this issue? why malloc(64K) is ok
> but malloc(128K) not? Is there any existing parameters about this
> issue? why not kernel attempt to shrunk cache no matter how big memory
> allocation is requested?
>
> Any thoughts?

The pattern you are seeing here is probably due to the page allocator
always retrying process context allocations which are <= order 3 (64K
with 4K pages).

You might be able to increase this limit a bit for your system, but it
could easily cause problems. Especially fragmentation on nommu systems
where the anonymous memory cannot be paged out.

--
SUSE Labs, Novell Inc.
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