Re: [Qemu-devel] [question] virtio-blk performance degradation happened with virito-serial

[Stefan Hajnoczi]

On Fri, Sep 12, 2014 at 11:21:37AM +0800, Zhang Haoyu wrote:
> >>> > > If virtio-blk and virtio-serial share an IRQ, the guest operating 
> >>> > > system has to check each virtqueue for activity. Maybe there is some 
> >>> > > inefficiency doing that.
> >>> > > AFAIK virtio-serial registers 64 virtqueues (on 31 ports + console) 
> >>> > > even if everything is unused.
> >>> > 
> >>> > That could be the case if MSI is disabled.
> >>> 
> >>> Do the windows virtio drivers enable MSIs, in their inf file?
> >>
> >>It depends on the version of the drivers, but it is a reasonable guess
> >>at what differs between Linux and Windows.  Haoyu, can you give us the
> >>output of lspci from a Linux guest?
> >>
> >I made a test with fio on rhel-6.5 guest, the same degradation happened too, 
> > this degradation can be reproduced on rhel6.5 guest 100%.
> >virtio_console module installed:
> >64K-write-sequence: 285 MBPS, 4380 IOPS
> >virtio_console module uninstalled:
> >64K-write-sequence: 370 MBPS, 5670 IOPS
> >
> I use top -d 1 -H -p <qemu-pid> to monitor the cpu usage, and found that,
> virtio_console module installed:
> qemu main thread cpu usage: 98%
> virtio_console module uninstalled:
> qemu main thread cpu usage: 60%
> 
> perf top -p <qemu-pid> result,
> virtio_console module installed:
>    PerfTop:    9868 irqs/sec  kernel:76.4%  exact:  0.0% [4000Hz cycles],  
> (target_pid: 88381)
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------
> 
>     11.80%  [kernel]                 [k] _raw_spin_lock_irqsave
>      8.42%  [kernel]                 [k] _raw_spin_unlock_irqrestore
>      7.33%  [kernel]                 [k] fget_light
>      6.28%  [kernel]                 [k] fput
>      3.61%  [kernel]                 [k] do_sys_poll
>      3.30%  qemu-system-x86_64       [.] qcow2_check_metadata_overlap
>      3.10%  [kernel]                 [k] __pollwait
>      2.15%  qemu-system-x86_64       [.] qemu_iohandler_poll
>      1.44%  libglib-2.0.so.0.3200.4  [.] g_array_append_vals
>      1.36%  libc-2.13.so             [.] 0x000000000011fc2a
>      1.31%  libpthread-2.13.so       [.] pthread_mutex_lock
>      1.24%  libglib-2.0.so.0.3200.4  [.] 0x000000000001f961
>      1.20%  libpthread-2.13.so       [.] __pthread_mutex_unlock_usercnt
>      0.99%  [kernel]                 [k] eventfd_poll
>      0.98%  [vdso]                   [.] 0x0000000000000771
>      0.97%  [kernel]                 [k] remove_wait_queue
>      0.96%  qemu-system-x86_64       [.] qemu_iohandler_fill
>      0.95%  [kernel]                 [k] add_wait_queue
>      0.69%  [kernel]                 [k] __srcu_read_lock
>      0.58%  [kernel]                 [k] poll_freewait
>      0.57%  [kernel]                 [k] _raw_spin_lock_irq
>      0.54%  [kernel]                 [k] __srcu_read_unlock
>      0.47%  [kernel]                 [k] copy_user_enhanced_fast_string
>      0.46%  [kvm_intel]              [k] vmx_vcpu_run
>      0.46%  [kvm]                    [k] vcpu_enter_guest
>      0.42%  [kernel]                 [k] tcp_poll
>      0.41%  [kernel]                 [k] system_call_after_swapgs
>      0.40%  libglib-2.0.so.0.3200.4  [.] g_slice_alloc
>      0.40%  [kernel]                 [k] system_call
>      0.38%  libpthread-2.13.so       [.] 0x000000000000e18d
>      0.38%  libglib-2.0.so.0.3200.4  [.] g_slice_free1
>      0.38%  qemu-system-x86_64       [.] address_space_translate_internal
>      0.38%  [kernel]                 [k] _raw_spin_lock
>      0.37%  qemu-system-x86_64       [.] phys_page_find
>      0.36%  [kernel]                 [k] get_page_from_freelist
>      0.35%  [kernel]                 [k] sock_poll
>      0.34%  [kernel]                 [k] fsnotify
>      0.31%  libglib-2.0.so.0.3200.4  [.] g_main_context_check
>      0.30%  [kernel]                 [k] do_direct_IO
>      0.29%  libpthread-2.13.so       [.] pthread_getspecific
> 
> virtio_console module uninstalled:
>    PerfTop:    9138 irqs/sec  kernel:71.7%  exact:  0.0% [4000Hz cycles],  
> (target_pid: 88381)
> ------------------------------------------------------------------------------------------------------------------------------
> 
>      5.72%  qemu-system-x86_64       [.] qcow2_check_metadata_overlap
>      4.51%  [kernel]                 [k] fget_light
>      3.98%  [kernel]                 [k] _raw_spin_lock_irqsave
>      2.55%  [kernel]                 [k] fput
>      2.48%  libpthread-2.13.so       [.] pthread_mutex_lock
>      2.46%  [kernel]                 [k] _raw_spin_unlock_irqrestore
>      2.21%  libpthread-2.13.so       [.] __pthread_mutex_unlock_usercnt
>      1.71%  [vdso]                   [.] 0x000000000000060c
>      1.68%  libc-2.13.so             [.] 0x00000000000e751f
>      1.64%  libglib-2.0.so.0.3200.4  [.] 0x000000000004fca0
>      1.20%  [kernel]                 [k] __srcu_read_lock
>      1.14%  [kernel]                 [k] do_sys_poll
>      0.96%  [kernel]                 [k] _raw_spin_lock_irq
>      0.95%  [kernel]                 [k] __pollwait
>      0.91%  [kernel]                 [k] __srcu_read_unlock
>      0.78%  [kernel]                 [k] tcp_poll
>      0.74%  [kvm]                    [k] vcpu_enter_guest
>      0.73%  [kvm_intel]              [k] vmx_vcpu_run
>      0.72%  [kernel]                 [k] _raw_spin_lock
>      0.72%  [kernel]                 [k] system_call_after_swapgs
>      0.70%  [kernel]                 [k] copy_user_enhanced_fast_string
>      0.67%  libglib-2.0.so.0.3200.4  [.] g_slice_free1
>      0.66%  libpthread-2.13.so       [.] 0x000000000000e12d
>      0.65%  [kernel]                 [k] system_call
>      0.61%  [kernel]                 [k] do_direct_IO
>      0.57%  qemu-system-x86_64       [.] qemu_iohandler_poll
>      0.57%  [kernel]                 [k] fsnotify
>      0.54%  libglib-2.0.so.0.3200.4  [.] g_slice_alloc
>      0.50%  [kernel]                 [k] vfs_write
>      0.49%  libpthread-2.13.so       [.] pthread_getspecific
>      0.48%  qemu-system-x86_64       [.] qemu_event_reset
>      0.47%  libglib-2.0.so.0.3200.4  [.] g_main_context_check
>      0.46%  qemu-system-x86_64       [.] address_space_translate_internal
>      0.46%  [kernel]                 [k] sock_poll
>      0.46%  libpthread-2.13.so       [.] __pthread_disable_asynccancel
>      0.44%  [kernel]                 [k] resched_task
>      0.43%  libpthread-2.13.so       [.] __pthread_enable_asynccancel
>      0.42%  qemu-system-x86_64       [.] phys_page_find
>      0.39%  qemu-system-x86_64       [.] object_dynamic_cast_assert

Max: Unrelated to this performance issue but I notice that the qcow2
metadata overlap check is high in the host CPU profile.  Have you had
any thoughts about optimizing the check?

Stefan

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