[pve-devel] cpuflag: pcid needed in guest for good performance after meltdown

Alexandre DERUMIER aderumier at odiso.com
Tue Jan 9 14:36:11 CET 2018 

>>i've the first customers having impacts. Current badest example: instead
>>of 12% cpu load 75% cpu load in guest.

Just curious, what kind of workload ?

I think with ceph and librbd, it'll hurt too for small block.

(I think I'll disable the patch cluster/osd side, as it's in a secure network, but client side will be impacted)

----- Mail original -----
De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag>
À: "aderumier" <aderumier at odiso.com>
Cc: "pve-devel" <pve-devel at pve.proxmox.com>
Envoyé: Mardi 9 Janvier 2018 14:32:27
Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown

Am 09.01.2018 um 14:24 schrieb Alexandre DERUMIER: 
> ok thanks ! 

i've the first customers having impacts. Current badest example: instead 
of 12% cpu load 75% cpu load in guest. 

> ----- Mail original ----- 
> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
> À: "aderumier" <aderumier at odiso.com> 
> Cc: "pve-devel" <pve-devel at pve.proxmox.com> 
> Envoyé: Mardi 9 Janvier 2018 14:10:27 
> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
> 
> Am 09.01.2018 um 13:02 schrieb Alexandre DERUMIER: 
>>>> You mean: 
>>>> -cpu qemu64,+pcid 
>> 
>> yes. 
>> 
>> (don't known the perf difference between qemu64 (without pcid), and intel cpu model (without pcid too) ? ) 
> 
> The results are nearly the same. 
> 
> Same VM as before now model qemu64,+pcid: 
> real 0m13.870s 
> user 0m7.128s 
> sys 0m6.697s 
> 
> qemu64: 
> real 0m25.214s 
> user 0m16.923s 
> sys 0m8.956s 
> 
> Stefan 
>> 
>> ----- Mail original ----- 
>> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
>> À: "aderumier" <aderumier at odiso.com> 
>> Cc: "pve-devel" <pve-devel at pve.proxmox.com> 
>> Envoyé: Mardi 9 Janvier 2018 12:57:30 
>> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>> 
>> Am 09.01.2018 um 12:55 schrieb Alexandre DERUMIER: 
>>>>> Yes - see an example which does a lot of syscalls: 
>>> 
>>> and for qemu64 ? (is it possible to sent +pcid too ?) 
>> 
>> You mean: 
>> -cpu qemu64,+pcid 
>> ? 
>> 
>> Stefan 
>> 
>>> ----- Mail original ----- 
>>> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
>>> À: "aderumier" <aderumier at odiso.com> 
>>> Cc: "pve-devel" <pve-devel at pve.proxmox.com> 
>>> Envoyé: Mardi 9 Janvier 2018 12:20:04 
>>> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>>> 
>>> Am 09.01.2018 um 10:43 schrieb Alexandre DERUMIER: 
>>>>>> That's bad as pcid is very important to performance for meltdown fixes 
>>>>>> in the linux kernel. 
>>>> 
>>>> I wonder the difference of performance for 
>>>> 
>>>> - qemu64|kvm64 cpu model 
>>>> - intel cpu model 
>>>> - intel + pcid cpu model ? 
>>>> 
>>>> (currently I'm running mainly qemu64 because I don't need advanced cpu flag) 
>>>> 
>>>> Do you have already done some benchmarks ? 
>>> 
>>> 
>>> Yes - see an example which does a lot of syscalls: 
>>> 
>>> no tasks running other than du: 
>>> 
>>> no pcid: 
>>> # time for i in $(seq 1 1 50); do du -sx /; done 
>>> ... 
>>> real 0m26.614s 
>>> user 0m17.548s 
>>> sys 0m9.056s 
>>> 
>>> 
>>> kvm started with +pcid: 
>>> # time for i in $(seq 1 1 50); do du -sx /; done 
>>> ... 
>>> real 0m14.734s 
>>> user 0m7.755s 
>>> sys 0m6.973s 
>>> 
>>> Greets, 
>>> Stefan 
>>> 
>>>> 
>>>> ----- Mail original ----- 
>>>> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
>>>> À: "aderumier" <aderumier at odiso.com> 
>>>> Cc: "pve-devel" <pve-devel at pve.proxmox.com> 
>>>> Envoyé: Mardi 9 Janvier 2018 10:20:39 
>>>> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>>>> 
>>>> Am 09.01.2018 um 09:18 schrieb Alexandre DERUMIER: 
>>>>> they are a discussion on qemu mailing currently about pcid 
>>>>> 
>>>>> 
>>>>> From paolo bonzini: 
>>>>> 
>>>>> " 
>>>>> Note that PCID is still not supported for guests without EPT, so 
>>>>> this would break ept=0 with recent "-cpu" models. I'm not sure of 
>>>>> a way to fix it; probably it just has to be documented." 
>>>> 
>>>> That's bad as pcid is very important to performance for meltdown fixes 
>>>> in the linux kernel. 
>>>> 
>>>> Stefan 
>>>> 
>>>>> 
>>>>> ----- Mail original ----- 
>>>>> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
>>>>> À: "pve-devel" <pve-devel at pve.proxmox.com>, "aderumier" <aderumier at odiso.com> 
>>>>> Envoyé: Mardi 9 Janvier 2018 08:35:00 
>>>>> Objet: Re: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>>>>> 
>>>>> Am 08.01.2018 um 23:23 schrieb Alexandre DERUMIER: 
>>>>>> I think it's not exposed in current cpu model 
>>>>>> 
>>>>>> it can be enabled with "+pcid" 
>>>>>> 
>>>>>> 
>>>>>> I don't known what it's the best way to add it in proxmox. 
>>>>>> We could do it with qemu version upgrade, but qemu 2.10 are not ready. 
>>>>> 
>>>>> Yes but that was always bad at least to me. We're limiting ourselfes if 
>>>>> we can only add new qemu features with a new qemu version. 
>>>>> 
>>>>> What about a version or feature field in the guest config which we only 
>>>>> update on a fresh vm start. 
>>>>> 
>>>>> Something like this: 
>>>>> 123.conf: 
>>>>> ... 
>>>>> pve_qemu_version: 2.11-10 
>>>>> ... 
>>>>> 
>>>>> This field is ONLY and ALWAYS updated in a fresh start not on migration. 
>>>>> This can be easily detected. 
>>>>> 
>>>>> Than we can do stuff like 
>>>>> if (version_cmp($conf->{pve_qemu_version}, "2.11-11")) { 
>>>>> # enable pcid flag 
>>>>> } 
>>>>> 
>>>>> Greets 
>>>>> Stefan 
>>>>> 
>>>>>> Maybe add new cpumodel with +pcid enabled ? 
>>>>>> or add code to manage custom cpuflags and add a checkbox in cpu options ? 
>>>>>> 
>>>>>> 
>>>>>> 
>>>>>> ----- Mail original ----- 
>>>>>> De: "Stefan Priebe, Profihost AG" <s.priebe at profihost.ag> 
>>>>>> À: "pve-devel" <pve-devel at pve.proxmox.com> 
>>>>>> Envoyé: Lundi 8 Janvier 2018 21:34:57 
>>>>>> Objet: [pve-devel] cpuflag: pcid needed in guest for good performance after meltdown 
>>>>>> 
>>>>>> Hello, 
>>>>>> 
>>>>>> for meltdown mitigation and performance it's important to have the pcid 
>>>>>> flag passed down to the guest (f.e. 
>>>>>> https://groups.google.com/forum/m/#!topic/mechanical-sympathy/L9mHTbeQLNU). 
>>>>>> 
>>>>>> My host shows the flag: 
>>>>>> # grep ' pcid ' /proc/cpuinfo | wc -l 
>>>>>> 56 
>>>>>> 
>>>>>> But the guest does not: 
>>>>>> # grep pcid /proc/cpuinfo 
>>>>>> # 
>>>>>> 
>>>>>> Guest was started with: 
>>>>>> -cpu IvyBridge,+kvm_pv_unhalt,+kvm_pv_eoi,enforce,vendor=GenuineIntel 
>>>>>> 
>>>>>> Is this something missing in host kernel or in PVE? 
>>>>>> 
>>>>>> Greets, 
>>>>>> Stefan 
>>>>>> _______________________________________________ 
>>>>>> pve-devel mailing list 
>>>>>> pve-devel at pve.proxmox.com 
>>>>>> https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel 
>>>>>> 
>>>>>> _______________________________________________ 
>>>>>> pve-devel mailing list 
>>>>>> pve-devel at pve.proxmox.com 
>>>>>> https://pve.proxmox.com/cgi-bin/mailman/listinfo/pve-devel 
>>>>>> 
>>>>> 
>>>> 
>>> 
>> 
>

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