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Re: [Qemu-devel] qemu core file size

From: Alexey Kardashevskiy
Subject: Re: [Qemu-devel] qemu core file size
Date: Tue, 7 Nov 2017 17:12:10 +1100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.4.0

On 07/11/17 17:02, Wanpeng Li wrote:
> Hi Alexey,
> 2017-11-07 13:46 GMT+08:00 Alexey Kardashevskiy <address@hidden>:
>> On 07/11/17 01:08, Paolo Bonzini wrote:
>>> On 06/11/2017 13:18, Wanpeng Li wrote:
>>>> 2017-11-06 20:02 GMT+08:00 Paolo Bonzini <address@hidden>:
>>>>> On 06/11/2017 12:59, Fam Zheng wrote:
>>>>>>>> Could you point out the patchset for the fix?
>>>>>>> Between 447b0d0b9ee8a0ac216c3186e0f3c427a1001f0c and
>>>>>>> 092aa2fc65b7a35121616aad8f39d47b8f921618.
>>>>>> Not sure how these relate to the core size, but I've tested upstream
>>>>>> (ec7a8bf0b8f7dc7288fe8745464ee8217528cc6c) and with dump-guest-core=off 
>>>>>> the core
>>>>>> file is 363M, still significantly larger than rss (~73M).
>>>>>> What is bloating the core file?
>>>>> My guess would have been fragmented heap.  The core file, unlike the
>>>>> RSS, includes all the mmaped memory (e.g. from shared libraries) that
>>>>> has never been used.
>>>>> For example, all the Ceph/Gluster/PulseAudio/SPICE/whatever libraries
>>>>> are included in the core file but likely are not in the RSS.
>>>> Do you mean not use Memory API will avoid the fragmented heap?
>>> The high memory usage from the memory API causes excessive
>>> fragmentation.  Alexey's work should help reducing memory usage and thus
>>> the fragmentation.
>> Since centos6 does not have this issue and centos7 does, I'd suggest
>> https://www.gnu.org/software/libc/manual/html_node/Memory-Allocation-Tunables.html
> Thanks for the inputs. What's the recommend glibc.malloc.arena_max
> value from you? :)

I have no idea, you have to try different values as you are the one who is
unhappy about the sizes :) I do not normally mess with this as things
should just work with the defaults whatever they are.

>> glibc changed some defaults between rhel/centos 6 and 7 if I recall
>> correctly, and these arenas now create big anon mappings per thread, these
>> are not really touched (so they do not use resident memory) but may appear
>> in these dumps, dunno.
>> btw do you configure the machine to make "kill -11" produce qemu core dumps?
> Yeah, some tuning in /etc/libvirt/qemu.conf, max_core="unlimited",
> dump_guest_core=0

dump_guest_core=0 should mean "no dumps", no?
Do you know how to enable this without libvirt?


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