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SubjectRe: [PATCH RFC 1/2] kvm: Handle undercommitted guest case in PLE handler
On 10/11/2012 12:57 AM, Andrew Theurer wrote:
> On Wed, 2012-10-10 at 23:13 +0530, Raghavendra K T wrote:
>> On 10/10/2012 07:54 PM, Andrew Theurer wrote:
>>> I ran 'perf sched map' on the dbench workload for medium and large VMs,
>>> and I thought I would share some of the results. I think it helps to
>>> visualize what's going on regarding the yielding.
>>>
>>> These files are png bitmaps, generated from processing output from 'perf
>>> sched map' (and perf data generated from 'perf sched record'). The Y
>>> axis is the host cpus, each row being 10 pixels high. For these tests,
>>> there are 80 host cpus, so the total height is 800 pixels. The X axis
>>> is time (in microseconds), with each pixel representing 1 microsecond.
>>> Each bitmap plots 30,000 microseconds. The bitmaps are quite wide
>>> obviously, and zooming in/out while viewing is recommended.
>>>
>>> Each row (each host cpu) is assigned a color based on what thread is
>>> running. vCPUs of the same VM are assigned a common color (like red,
>>> blue, magenta, etc), and each vCPU has a unique brightness for that
>>> color. There are a maximum of 12 assignable colors, so in any VMs >12
>>> revert to vCPU color of gray. I would use more colors, but it becomes
>>> harder to distinguish one color from another. The white color
>>> represents missing data from perf, and black color represents any thread
>>> which is not a vCPU.
>>>
>>> For the following tests, VMs were pinned to host NUMA nodes and to
>>> specific cpus to help with consistency and operate within the
>>> constraints of the last test (gang scheduler).
>>>
>>> Here is a good example of PLE. These are 10-way VMs, 16 of them (as
>>> described above only 12 of the VMs have a color, rest are gray).
>>>
>>> https://docs.google.com/open?id=0B6tfUNlZ-14wdmFqUmE5QjJHMFU
>>
>> This looks very nice to visualize what is happening. Beginning of the
>> graph looks little messy but later it is clear.
>>
>>>
>>> If you zoom out and look at the whole bitmap, you may notice the 4ms
>>> intervals of the scheduler. They are pretty well aligned across all
>>> cpus. Normally, for cpu bound workloads, we would expect to see each
>>> thread to run for 4 ms, then something else getting to run, and so on.
>>> That is mostly true in this test. We have 2x over-commit and we
>>> generally see the switching of threads at 4ms. One thing to note is
>>> that not all vCPU threads for the same VM run at exactly the same time,
>>> and that is expected and the whole reason for lock-holder preemption.
>>> Now, if you zoom in on the bitmap, you should notice within the 4ms
>>> intervals there is some task switching going on. This is most likely
>>> because of the yield_to initiated by the PLE handler. In this case
>>> there is not that much yielding to do. It's quite clean, and the
>>> performance is quite good.
>>>
>>> Below is an example of PLE, but this time with 20-way VMs, 8 of them.
>>> CPU over-commit is still 2x.
>>>
>>> https://docs.google.com/open?id=0B6tfUNlZ-14wdmFqUmE5QjJHMFU
>>
>> I think this link still 10x16. Could you paste the link again?
>
> Oops
> https://docs.google.com/open?id=0B6tfUNlZ-14wSGtYYzZtRTcyVjQ
>
>>
>>>
>>> This one looks quite different. In short, it's a mess. The switching
>>> between tasks can be lower than 10 microseconds. It basically never
>>> recovers. There is constant yielding all the time.
>>>
>>> Below is again 8 x 20-way VMs, but this time I tried out Nikunj's gang
>>> scheduling patches. While I am not recommending gang scheduling, I
>>> think it's a good data point. The performance is 3.88x the PLE result.
>>>
>>> https://docs.google.com/open?id=0B6tfUNlZ-14wWXdscWcwNTVEY3M

Yes.. we see lot of yields.



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