How virtio-blk latency can be measured

How virtio-blk latency can be measured

[Qingshu Chen]

hi,I want to calculate the latencyof virtio in kvm, I read the document
in http://www.linux-kvm.org/page/Virtio/Block/Latency and met some
problem.
1. when calculating latency in kvm, the document said  kvm_pio and
kvm_set_irq can be filtered using command 'lspci -vv -nn' and
'cat /proc/interrupts' , but I don't know how to use the result to
filter kvm_pio and kvm_set_irq.
2. Anyone know who is the author of this webpage:
http://www.linux-kvm.org/page/Virtio/Block/Latency .
Thank you.

How virtio-blk latency can be measured

[Qingshu Chen]

hi,I want to calculate the latencyof virtio in kvm, I read the document in
http://www.linux-kvm.org/page/Virtio/Block/Latency and met some problem.
1. when calculating latency in kvm, the document said  kvm_pio and
kvm_set_irq can be filtered using command 'lspci -vv -nn' and 'cat
/proc/interrupts' , but I don't know how to use the result to filter kvm_pio
and kvm_set_irq.
2. Anyone know who is the author of this webpage:
http://www.linux-kvm.org/page/Virtio/Block/Latency .
Thank you

Re: How virtio-blk latency can be measured

[Stefan Hajnoczi]

On Sun, Dec 15, 2013 at 04:20:05PM +0800, Qingshu Chen wrote:
> hi,I want to calculate the latencyof virtio in kvm, I read the document in
> http://www.linux-kvm.org/page/Virtio/Block/Latency and met some problem.
> 1. when calculating latency in kvm, the document said  kvm_pio and
> kvm_set_irq can be filtered using command 'lspci -vv -nn' and 'cat
> /proc/interrupts' , but I don't know how to use the result to filter kvm_pio
> and kvm_set_irq.

Hi Qingshu,
I wrote that wiki page.  The commands on the wiki are:

  cd /sys/kernel/debug/tracing
  echo 'port == 0xc090' >events/kvm/kvm_pio/filter
  echo 'gsi == 26' >events/kvm/kvm_set_irq/filter
  echo 1 >events/kvm/kvm_pio/enable
  echo 1 >events/kvm/kvm_set_irq/enable
  cat trace_pipe >/tmp/trace

Here is how you can find the right port and gsi values to filter:

1. The 'kvm_pio' event is used to trace guest->host virtqueue
   notification.  Looking at drivers/virtio/virtio_pci.c reveals that
   the port is ioaddr + VIRTIO_PCI_QUEUE_NOTIFY (16).

   Here is an example:
   # lspci -vv -nn
   00:04.0 SCSI storage controller [0100]: Red Hat, Inc Virtio block device [1af4:1001]
   [...]
      Region 0: I/O ports at c080 [size=64]

   Therefore you need to filter on 0xc080 + 0x10 = 0xc090 to trace
   VIRTIO_PCI_QUEUE_NOTIFY accesses.

2. The 'kvm_set_irq' event is used to trace host->guest virtqueue
   notifications.

   Find the virtio driver instance:
   # ls /sys/block/vda/device/driver/
   bind  module  unbind  virtio1

   Now look up the virtqueue interrupt for 'virtio1':
   # cat /proc/interrupts
   [...]
   185:      35129          0          0          0          0 0       8760          0       PCI-MSI-X  virtio1-requests

   So gsi is 185.

Stefan