Messages in this thread | | | Date | Wed, 04 Jan 2012 11:47:17 -0500 | From | Rik van Riel <> | Subject | Re: [RFC PATCH 0/4] Gang scheduling in CFS |
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On 01/04/2012 09:41 AM, Avi Kivity wrote: > On 01/04/2012 12:52 PM, Nikunj A Dadhania wrote: >> On Mon, 02 Jan 2012 11:37:22 +0200, Avi Kivity<avi@redhat.com> wrote: >>> On 12/31/2011 04:21 AM, Nikunj A Dadhania wrote: >>>> >>>> GangV2: >>>> 27.45% ebizzy libc-2.12.so [.] __memcpy_ssse3_back >>>> 12.12% ebizzy [kernel.kallsyms] [k] clear_page >>>> 9.22% ebizzy [kernel.kallsyms] [k] __do_page_fault >>>> 6.91% ebizzy [kernel.kallsyms] [k] flush_tlb_others_ipi >>>> 4.06% ebizzy [kernel.kallsyms] [k] get_page_from_freelist >>>> 4.04% ebizzy [kernel.kallsyms] [k] ____pagevec_lru_add >>>> >>>> GangBase: >>>> 45.08% ebizzy [kernel.kallsyms] [k] flush_tlb_others_ipi >>>> 15.38% ebizzy libc-2.12.so [.] __memcpy_ssse3_back >>>> 7.00% ebizzy [kernel.kallsyms] [k] clear_page >>>> 4.88% ebizzy [kernel.kallsyms] [k] __do_page_fault >>> >>> Looping in flush_tlb_others(). Rik, what trace an we run to find out >>> why PLE directed yield isn't working as expected? >>> >> I tried some experiments by adding a pause_loop_exits stat in the >> kvm_vpu_stat. > > (that's deprecated, we use tracepoints these days for stats) > >> Here are some observation related to Baseline-only(8vm case) >> >> | ple_gap=128 | ple_gap=64 | ple_gap=256 | ple_window=2048 >> --------------+-------------+------------+-------------+---------------- >> EbzyRecords/s | 2247.50 | 2132.75 | 2086.25 | 1835.62 >> PauseExits | 7928154.00 | 6696342.00 | 7365999.00 | 50319582.00 >> >> With ple_window = 2048, PauseExits is more than 6times the default case > > So it looks like the default is optimal, at least wrt the cases you > tested and your test workload.
It depends on the workload.
I believe ebizzy synchronously bounces messages around between userland threads, and may benefit from lower latency preemption and re-scheduling.
Workloads like AMQP do asynchronous messaging, and are likely to benefit from having a lower number of switches.
I do not know which kind of workload is more prevalent.
Another worry with gang scheduling is scalability. One of the reasons Linux scales well to larger systems is that a lot of things are done CPU local, without communicating things with other CPUs. Making the scheduling algorithm system-global has the potential to add in a lot of overhead.
Likewise, removing the ability to migrate workloads to idle CPUs is likely to hurt a lot of real world workloads.
Benchmarks don't care, because they run full-out. However, users do not run benchmarks nearly as much as they run actual workloads...
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