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 > > > > Note that the task switching intervals of 4ms are quite obvious again, > > and this time all vCPUs from same VM run at the same time. It > > represents the best possible outcome. > > > > > > Anyway, I thought the bitmaps might help better visualize what's going > > on. > > > > -Andrew > > > > > > > > > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
