[libvirt] [RFC PATCHv2 00/10] x86 RDT Cache Monitoring Technology (CMT)

Wed Jul 18 12:07:17 UTC 2018

On Wed, Jul 18, 2018 at 02:29:32AM +0000, Wang, Huaqiang wrote:
>
>
>> -----Original Message-----
>> From: Martin Kletzander [mailto:mkletzan at redhat.com]
>> Sent: Tuesday, July 17, 2018 5:11 PM
>> To: Wang, Huaqiang <huaqiang.wang at intel.com>
>> Cc: libvir-list at redhat.com; Feng, Shaohe <shaohe.feng at intel.com>; Niu, Bing
>> <bing.niu at intel.com>; Ding, Jian-feng <jian-feng.ding at intel.com>; Zang, Rui
>> <rui.zang at intel.com>
>> Subject: Re: [libvirt] [RFC PATCHv2 00/10] x86 RDT Cache Monitoring
>> Technology (CMT)
>>
>> On Tue, Jul 17, 2018 at 07:19:41AM +0000, Wang, Huaqiang wrote:
>> >Hi Martin,
>> >
>> >Thanks for your comments. Please see my reply inline.
>> >
>> >> -----Original Message-----
>> >> From: Martin Kletzander [mailto:mkletzan at redhat.com]
>> >> Sent: Tuesday, July 17, 2018 2:27 PM
>> >> To: Wang, Huaqiang <huaqiang.wang at intel.com>
>> >> Cc: libvir-list at redhat.com; Feng, Shaohe <shaohe.feng at intel.com>;
>> >> Niu, Bing <bing.niu at intel.com>; Ding, Jian-feng
>> >> <jian-feng.ding at intel.com>; Zang, Rui <rui.zang at intel.com>
>> >> Subject: Re: [libvirt] [RFC PATCHv2 00/10] x86 RDT Cache Monitoring
>> >> Technology (CMT)
>> >>
>> >> On Mon, Jul 09, 2018 at 03:00:48PM +0800, Wang Huaqiang wrote:
>> >> >
>> >> >This is the V2 of RFC and the POC source code for introducing x86
>> >> >RDT CMT feature, thanks Martin Kletzander for his review and
>> >> >constructive suggestion for V1.
>> >> >
>> >> >This series is trying to provide the similar functions of the perf
>> >> >event based CMT, MBMT and MBML features in reporting cache
>> >> >occupancy, total memory bandwidth utilization and local memory
>> >> >bandwidth utilization information in livirt. Firstly we focus on cmt.
>> >> >
>> >> >x86 RDT Cache Monitoring Technology (CMT) provides a medthod to
>> >> >track the cache occupancy information per CPU thread. We are
>> >> >leveraging the implementation of kernel resctrl filesystem and
>> >> >create our patches on top of that.
>> >> >
>> >> >Describing the functionality from a high level:
>> >> >
>> >> >1. Extend the output of 'domstats' and report CMT inforamtion.
>> >> >
>> >> >Comparing with perf event based CMT implementation in libvirt, this
>> >> >series extends the output of command 'domstat' and reports cache
>> >> >occupancy information like these:
>> >> ><pre>
>> >> >[root at dl-c200 libvirt]# virsh domstats vm3 --cpu-resource
>> >> >Domain: 'vm3'
>> >> >  cpu.cacheoccupancy.vcpus_2.value=4415488
>> >> >  cpu.cacheoccupancy.vcpus_2.vcpus=2
>> >> >  cpu.cacheoccupancy.vcpus_1.value=7839744
>> >> >  cpu.cacheoccupancy.vcpus_1.vcpus=1
>> >> >  cpu.cacheoccupancy.vcpus_0,3.value=53796864
>> >> >  cpu.cacheoccupancy.vcpus_0,3.vcpus=0,3
>> >> ></pre>
>> >> >The vcpus have been arragned into three monitoring groups, these
>> >> >three groups cover vcpu 1, vcpu 2 and vcpus 0,3 respectively. Take
>> >> >an example, the 'cpu.cacheoccupancy.vcpus_0,3.value' reports the
>> >> >cache occupancy information for vcpu 0 and vcpu 3, the
>> >> 'cpu.cacheoccupancy.vcpus_0,3.vcpus'
>> >> >represents the vcpu group information.
>> >> >
>> >> >To address Martin's suggestion "beware as 1-4 is something else than
>> >> >1,4 so you need to differentiate that.", the content of 'vcpus'
>> >> >(cpu.cacheoccupancy.<groupname>.vcpus=xxx) has been specially
>> >> >processed, if vcpus is a continous range, e.g. 0-2, then the output
>> >> >of cpu.cacheoccupancy.vcpus_0-2.vcpus will be like
>> >> >'cpu.cacheoccupancy.vcpus_0-2.vcpus=0,1,2'
>> >> >instead of
>> >> >'cpu.cacheoccupancy.vcpus_0-2.vcpus=0-2'.
>> >> >Please note that 'vcpus_0-2' is a name of this monitoring group,
>> >> >could be specified any other word from the XML configuration file or
>> >> >lively changed with the command introduced in following part.
>> >> >
>> >>
>> >> One small nit according to the naming (but it shouldn't block any
>> >> reviewers from reviewing, just keep this in mind for next version for
>> >> example) is that this is still inconsistent.
>> >
>> >OK.  I'll try to use words such as 'cache', 'cpu resource' and avoid
>> >using 'RDT', 'CMT'.
>> >
>>
>> Oh, you misunderstood, I meant the naming in the domstats output =)
>>
>> >The way domstats are structured when there is something like an
>> >> array could shed some light into this.  What you suggested is really
>> >> kind of hard to parse (although looks better).  What would you say to
>> something like this:
>> >>
>> >>   cpu.cacheoccupancy.count = 3
>> >>   cpu.cacheoccupancy.0.value=4415488
>> >>   cpu.cacheoccupancy.0.vcpus=2
>> >>   cpu.cacheoccupancy.0.name=vcpus_2
>> >>   cpu.cacheoccupancy.1.value=7839744
>> >>   cpu.cacheoccupancy.1.vcpus=1
>> >>   cpu.cacheoccupancy.1.name=vcpus_1
>> >>   cpu.cacheoccupancy.2.value=53796864
>> >>   cpu.cacheoccupancy.2.vcpus=0,3
>> >>   cpu.cacheoccupancy.2.name=0,3
>> >>
>> >
>> >Your arrangement looks more reasonable, thanks for your advice.
>> >However, as I mentioned in another email that I sent to libvirt-list
>> >hours ago, the kernel resctrl interface provides cache occupancy
>> >information for each cache block for every resource group.
>> >Maybe we need to expose the cache occupancy for each cache block.
>> >If you agree, we need to refine the 'domstats' output message, how
>> >about this:
>> >
>> >  cpu.cacheoccupancy.count=3
>> >  cpu.cacheoccupancy.0.name=vcpus_2
>> >  cpu.cacheoccupancy.0.vcpus=2
>> >  cpu.cacheoccupancy.0.block.count=2
>> >  cpu.cacheoccupancy.0.block.0.bytes=5488
>> >  cpu.cacheoccupancy.0.block.1. bytes =4410000
>> >  cpu.cacheoccupancy.1.name=vcpus_1
>> >  cpu.cacheoccupancy.1.vcpus=1
>> >  cpu.cacheoccupancy.1.block.count=2
>> >  cpu.cacheoccupancy.1.block.0. bytes =7839744
>> > cpu.cacheoccupancy.1.block.0. bytes =0
>> >  cpu.cacheoccupancy.2.name=0,3
>> >  cpu.cacheoccupancy.2.vcpus=0,3
>> >  cpu.cacheoccupancy.2.block.count=2
>> >  cpu.cacheoccupancy.2.block.0. bytes=53796864
>> > cpu.cacheoccupancy.2.block.1. bytes=0
>> >
>>
>> What do you mean by cache block?  Is that (cache_size / granularity)?  In that
>> case it looks fine, I guess (without putting too much thought into it).
>
>No. 'cache block' that I mean is indexed with 'cache id', with the id number
>kept in '/sys/devices/system/cpu/cpu*/cache/index*/id'.
>
>Generally for a two socket server  node, there are two sockets (with CPU
>E5-2680 v4, for example) in system, and each socket has a L3 cache,
>if resctrl monitoring group is created (/sys/fs/resctrl/p0, for example),
>you can find the cache occupancy information for these two L3 cache
>areas separately from file
>/sys/fs/resctrl/p0/mon_data/mon_L3_00/llc_occupancy
>and file
>/sys/fs/resctrl/p0/mon_data/mon_L3_01/llc_occupancy
>Cache information for individual socket is meaningful to detect performance
>issues such as workload balancing...etc. We'd better expose these details to
>libvirt users.
>To my knowledge, I am using 'cache block' to describe the CPU cache
>indexed with number found in '/sys/devices/system/cpu/cpu*/cache/index*/id'.
>I welcome suggestion on other kind of naming for it.
>

To be consistent I'd prefer "cache" "cache bank" and "index" or "id".  I don't
have specific requirements, I just don't want to invent new words.  Look at how
it is described in capabilities for example.

>>
>> Martin
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