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

[Nick Piggin]

Aubrey wrote:
> On 11/29/06, Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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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