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Subject: Re: [Qemu-devel] [PATCH] Permit zero-sized qemu_malloc() & friends
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To: malc <av1474@comtv.ru>
Cc: Blue Swirl <blauwirbel@gmail.com>, qemu-devel@nongnu.org, Ian Molton <ian.molton@collabora.co.uk>, Paul Brook <paul@codesourcery.com>, Markus Armbruster <armbru@redhat.com>

On 12/06/2009 07:47 PM, malc wrote:
>
>> It will never fail on Linux.  On other hosts it prevents a broken oom handler
>> from taking the guest down a death spiral.
>>      
> It fails here all the time i'm sorry to say, i have overcommit disabled
> (mostly because kpdf when doing a text search tends to overwhelm the VM
> subsystem and Linux happily picks X11 as it's OOM kill target)
>    

Right, I meant under the default configuration.  Unfortunately there is 
no good configuration for Linux - without strict overcommit you'll 
invoke the oom killer, with strict overcommit you'll need ridiculous 
amounts of swap because fork() and MAP_PRIVATE require tons of backing 
store.

On my home machine I have

   $ grep Commit /proc/meminfo
   CommitLimit:     7235160 kB
   Committed_AS:    4386172 kB

So, 4GB of virtual memory needed just to run a normal desktop with 
thunderbird+firefox.  Actual anonymous memory is less than 2GB, so you 
could easily run this workload on a 2GB machine without swap, but with 
strict overcommit 2GB RAM + 2GB swap will see failures even though you 
have free RAM.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.