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

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Message-ID: <456F4A95.2090503@yahoo.com.au>
Date:	Fri, 01 Dec 2006 08:18:13 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Aubrey <aubreylee@...il.com>
CC:	Sonic Zhang <sonic.adi@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	vapier.adi@...il.com
Subject: Re: The VFS cache is not freed when there is not enough free memory
 to allocate

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