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

From: Nick Piggin
Date: Thu Nov 30 2006 - 16:19:31 EST


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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