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[for-6.1 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-bl
From: |
Greg Kurz |
Subject: |
[for-6.1 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci |
Date: |
Wed, 7 Apr 2021 16:34:57 +0200 |
Now that virtio-scsi-pci and virtio-blk-pci map 1 virtqueue per vCPU,
a serious slow down may be observed on setups with a big enough number
of vCPUs.
Exemple with a pseries guest on a bi-POWER9 socket system (128 HW threads):
virtio-scsi virtio-blk
1 0m20.922s 0m21.346s
2 0m21.230s 0m20.350s
4 0m21.761s 0m20.997s
8 0m22.770s 0m20.051s
16 0m22.038s 0m19.994s
32 0m22.928s 0m20.803s
64 0m26.583s 0m22.953s
128 0m41.273s 0m32.333s
256 2m4.727s 1m16.924s
384 6m5.563s 3m26.186s
Both perf and gprof indicate that QEMU is hogging CPUs when setting up
the ioeventfds:
67.88% swapper [kernel.kallsyms] [k] power_pmu_enable
9.47% qemu-kvm [kernel.kallsyms] [k] smp_call_function_single
8.64% qemu-kvm [kernel.kallsyms] [k] power_pmu_enable
=>2.79% qemu-kvm qemu-kvm [.] memory_region_ioeventfd_before
=>2.12% qemu-kvm qemu-kvm [.] address_space_update_ioeventfds
0.56% kworker/8:0-mm [kernel.kallsyms] [k] smp_call_function_single
address_space_update_ioeventfds() is called when committing an MR
transaction, i.e. for each ioeventfd with the current code base,
and it internally loops on all ioventfds:
static void address_space_update_ioeventfds(AddressSpace *as)
{
[...]
FOR_EACH_FLAT_RANGE(fr, view) {
for (i = 0; i < fr->mr->ioeventfd_nb; ++i) {
This means that the setup of ioeventfds for these devices has
quadratic time complexity.
This series simply changes the device models to extend the transaction
to all virtqueueues, like already done in the past in the generic
code with 710fccf80d78 ("virtio: improve virtio devices initialization
time").
Only virtio-scsi and virtio-blk are covered here, but a similar change
might also be beneficial to other device types such as host-scsi-pci,
vhost-user-scsi-pci and vhost-user-blk-pci.
virtio-scsi virtio-blk
1 0m21.271s 0m22.076s
2 0m20.912s 0m19.716s
4 0m20.508s 0m19.310s
8 0m21.374s 0m20.273s
16 0m21.559s 0m21.374s
32 0m22.532s 0m21.271s
64 0m26.550s 0m22.007s
128 0m29.115s 0m27.446s
256 0m44.752s 0m41.004s
384 1m2.884s 0m58.023s
This should fix https://bugzilla.redhat.com/show_bug.cgi?id=1927108
which reported the issue for virtio-scsi-pci.
Changes since RFC:
As suggested by Stefan, splimplify the code by directly beginning and
committing the memory transaction from the device model, without all
the virtio specific proxying code and no changes needed in the memory
subsystem.
Greg Kurz (4):
virtio-blk: Fix rollback path in virtio_blk_data_plane_start()
virtio-blk: Configure all host notifiers in a single MR transaction
virtio-scsi: Set host notifiers and callbacks separately
virtio-scsi: Configure all host notifiers in a single MR transaction
hw/block/dataplane/virtio-blk.c | 36 +++++++++++++++++++--
hw/scsi/virtio-scsi-dataplane.c | 56 ++++++++++++++++++++++-----------
2 files changed, 72 insertions(+), 20 deletions(-)
--
2.26.3
- [for-6.1 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci,
Greg Kurz <=
- [for-6.1 1/4] virtio-blk: Fix rollback path in virtio_blk_data_plane_start(), Greg Kurz, 2021/04/07
- [for-6.1 2/4] virtio-blk: Configure all host notifiers in a single MR transaction, Greg Kurz, 2021/04/07
- [for-6.1 3/4] virtio-scsi: Set host notifiers and callbacks separately, Greg Kurz, 2021/04/07
- [for-6.1 4/4] virtio-scsi: Configure all host notifiers in a single MR transaction, Greg Kurz, 2021/04/07
- Re: [for-6.1 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Greg Kurz, 2021/04/19
- Re: [for-6.1 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Michael S. Tsirkin, 2021/04/19