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Re: [Qemu-devel] [PATCH 03/15] coroutine-ucontext: reduce stack size to

From: Peter Lieven
Subject: Re: [Qemu-devel] [PATCH 03/15] coroutine-ucontext: reduce stack size to 64kB
Date: Tue, 28 Jun 2016 13:17:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.8.0

Am 28.06.2016 um 12:57 schrieb Dr. David Alan Gilbert:
* Paolo Bonzini (address@hidden) wrote:

On 28/06/2016 11:01, Peter Lieven wrote:
evaluation with the recently introduced maximum stack size monitoring revealed
that the actual used stack size was never above 4kB so allocating 1MB stack
for each coroutine is a lot of wasted memory. So reduce the stack size to
64kB which should still give enough head room.
If we make the stack this much smaller, there is a non-zero chance of
smashing it.  You must add a guard page if you do this (actually more
than one because QEMU will happily have stack frames as big as 16 KB).
The stack counts for RSS but it's not actually allocated memory, so why
does it matter?
I think I'd be interested in seeing the /proc/.../smaps before and after this
change to see if anything is visible and if we can see the difference
in rss etc.

Can you advise what in smaps should be especially looked at.

As for RSS I can report hat the long term usage is significantly lower.
I had the strange observation that when the VM is running for some minutes
the RSS suddenly increases to the whole stack size.


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