• Jitendra Kolhe's avatar
    mem-prealloc: reduce large guest start-up and migration time. · 1e356fc1
    Jitendra Kolhe authored
    Using "-mem-prealloc" option for a large guest leads to higher guest
    start-up and migration time. This is because with "-mem-prealloc" option
    qemu tries to map every guest page (create address translations), and
    make sure the pages are available during runtime. virsh/libvirt by
    default, seems to use "-mem-prealloc" option in case the guest is
    configured to use huge pages. The patch tries to map all guest pages
    simultaneously by spawning multiple threads. Currently limiting the
    change to QEMU library functions on POSIX compliant host only, as we are
    not sure if the problem exists on win32. Below are some stats with
    "-mem-prealloc" option for guest configured to use huge pages.
    Idle Guest      | Start-up time | Migration time
    Guest stats with 2M HugePage usage - single threaded (existing code)
    64 Core - 4TB   | 54m11.796s    | 75m43.843s
    64 Core - 1TB   | 8m56.576s     | 14m29.049s
    64 Core - 256GB | 2m11.245s     | 3m26.598s
    Guest stats with 2M HugePage usage - map guest pages using 8 threads
    64 Core - 4TB   | 5m1.027s      | 34m10.565s
    64 Core - 1TB   | 1m10.366s     | 8m28.188s
    64 Core - 256GB | 0m19.040s     | 2m10.148s
    Guest stats with 2M HugePage usage - map guest pages using 16 threads
    64 Core - 4TB   | 1m58.970s     | 31m43.400s
    64 Core - 1TB   | 0m39.885s     | 7m55.289s
    64 Core - 256GB | 0m11.960s     | 2m0.135s
    Changed in v2:
     - modify number of memset threads spawned to min(smp_cpus, 16).
     - removed 64GB memory restriction for spawning memset threads.
    Changed in v3:
     - limit number of threads spawned based on
       min(sysconf(_SC_NPROCESSORS_ONLN), 16, smp_cpus)
     - implement memset thread specific siglongjmp in SIGBUS signal_handler.
    Changed in v4
     - remove sigsetjmp/siglongjmp and SIGBUS unblock/block for main thread
       as main thread no longer touches any pages.
     - simplify code my returning memset_thread_failed status from
    Signed-off-by: 's avatarJitendra Kolhe <jitendra.kolhe@hpe.com>
    Message-Id: <1487907103-32350-1-git-send-email-jitendra.kolhe@hpe.com>
    Signed-off-by: 's avatarPaolo Bonzini <pbonzini@redhat.com>
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