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Re: [Qemu-devel] qemu core file size

From: Wanpeng Li
Subject: Re: [Qemu-devel] qemu core file size
Date: Tue, 7 Nov 2017 14:16:11 +0800

2017-11-07 14:12 GMT+08:00 Alexey Kardashevskiy <address@hidden>:
> 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?

Please add dump_guest_core=off in your qemu command-line,
"dump_guest_core" mean not dump guest memory and just dump qemu

Wanpeng Li

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