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[Qemu-devel] [PATCH 0/7] virtio: virtio-blk data plane


From: Stefan Hajnoczi
Subject: [Qemu-devel] [PATCH 0/7] virtio: virtio-blk data plane
Date: Thu, 15 Nov 2012 16:18:59 +0100

This series adds the -device virtio-blk-pci,x-data-plane=on property that
enables a high performance I/O codepath.  A dedicated thread is used to process
virtio-blk requests outside the global mutex and without going through the QEMU
block layer.

Khoa Huynh <address@hidden> reported an increase from 140,000 IOPS to 600,000
IOPS for a single VM using virtio-blk-data-plane in July:

  http://comments.gmane.org/gmane.comp.emulators.kvm.devel/94580

The virtio-blk-data-plane approach was originally presented at Linux Plumbers
Conference 2010.  The following slides contain a brief overview:

  
http://linuxplumbersconf.org/2010/ocw/system/presentations/651/original/Optimizing_the_QEMU_Storage_Stack.pdf

The basic approach is:
1. Each virtio-blk device has a thread dedicated to handling ioeventfd
   signalling when the guest kicks the virtqueue.
2. Requests are processed without going through the QEMU block layer using
   Linux AIO directly.
3. Completion interrupts are injected via irqfd from the dedicated thread.

To try it out:

  qemu -drive if=none,id=drive0,cache=none,aio=native,format=raw,file=...
       -device virtio-blk-pci,drive=drive0,scsi=off,x-data-plane=on

Limitations:
 * Only format=raw is supported
 * Live migration is not supported
 * Block jobs, hot unplug, and other operations fail with -EBUSY
 * I/O throttling limits are ignored
 * Only Linux hosts are supported due to Linux AIO usage

The code has reached a stage where I feel it is ready to merge.  Users have
been playing with it for some time and want the significant performance boost.

We are refactoring QEMU to get rid of the global mutex.  I believe that
virtio-blk-data-plane can eventually become the default mode of operation.

Instead of waiting for global mutex removal efforts to finish, I want to use
virtio-blk-data-plane as an example device for AioContext and threaded hw
dispatch refactoring.  This means:

1. When the block layer can bind to an AioContext and execute I/O outside the
   global mutex, virtio-blk-data-plane can use this (and gain image format
   support).

2. When hw dispatch no longer needs the global mutex we can use hw/virtio.c
   again and perhaps run a pool of iothreads instead of dedicated data plane
   threads.

But in the meantime, I have cleaned up the virtio-blk-data-plane code so that
it can be merged as an experimental feature.

Changes from the RFC v9:
 * Add x-data-plane=on|off option and coexist with regular virtio-blk code
 * Create thread from BH so it inherits iothread cpusets
 * Drain requests on vm_stop() so stopped guest does not access image file
 * Add migration blocker
 * Add bdrv_in_use() to prevent block jobs and other operations that can 
interfere
 * Drop IOQueue request merging for simplicity
 * Drop ioctl interrupt injection and always use irqfd for simplicity
 * Major cleanup to split up source files
 * Rebase from qemu-kvm.git onto qemu.git
 * Address Michael Tsirkin's review comments

Stefan Hajnoczi (7):
  raw-posix: add raw_get_aio_fd() for virtio-blk-data-plane
  configure: add CONFIG_VIRTIO_BLK_DATA_PLANE
  dataplane: add virtqueue vring code
  dataplane: add event loop
  dataplane: add Linux AIO request queue
  dataplane: add virtio-blk data plane code
  virtio-blk: add x-data-plane=on|off performance feature

 block.h                    |   9 +
 block/raw-posix.c          |  34 ++++
 configure                  |  21 +++
 hw/Makefile.objs           |   2 +-
 hw/dataplane/Makefile.objs |   3 +
 hw/dataplane/event-poll.c  | 109 ++++++++++++
 hw/dataplane/event-poll.h  |  40 +++++
 hw/dataplane/ioq.c         | 118 +++++++++++++
 hw/dataplane/ioq.h         |  57 +++++++
 hw/dataplane/virtio-blk.c  | 414 +++++++++++++++++++++++++++++++++++++++++++++
 hw/dataplane/virtio-blk.h  |  41 +++++
 hw/dataplane/vring.c       | 321 +++++++++++++++++++++++++++++++++++
 hw/dataplane/vring.h       |  54 ++++++
 hw/virtio-blk.c            |  59 ++++++-
 hw/virtio-blk.h            |   1 +
 hw/virtio-pci.c            |   3 +
 trace-events               |   9 +
 17 files changed, 1293 insertions(+), 2 deletions(-)
 create mode 100644 hw/dataplane/Makefile.objs
 create mode 100644 hw/dataplane/event-poll.c
 create mode 100644 hw/dataplane/event-poll.h
 create mode 100644 hw/dataplane/ioq.c
 create mode 100644 hw/dataplane/ioq.h
 create mode 100644 hw/dataplane/virtio-blk.c
 create mode 100644 hw/dataplane/virtio-blk.h
 create mode 100644 hw/dataplane/vring.c
 create mode 100644 hw/dataplane/vring.h

-- 
1.8.0




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