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Re: [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virt
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci |
Date: |
Mon, 17 May 2021 14:37:17 +0100 |
On Mon, May 17, 2021 at 10:32:59AM +0200, Greg Kurz wrote:
> On Wed, 12 May 2021 17:05:53 +0100
> Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> > On Fri, May 07, 2021 at 06:59:01PM +0200, Greg Kurz wrote:
> > > 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 v1:
> > > - Add some comments (Stefan)
> > > - Drop optimization on the error path in patch 2 (Stefan)
> > >
> > > 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 | 45 ++++++++++++++++++++-
> > > hw/scsi/virtio-scsi-dataplane.c | 72 ++++++++++++++++++++++++---------
> > > 2 files changed, 97 insertions(+), 20 deletions(-)
> > >
> > > --
> > > 2.26.3
> > >
> >
> > Thanks, applied to my block tree:
> > https://gitlab.com/stefanha/qemu/commits/block
> >
>
> Hi Stefan,
>
> It seems that Michael already merged the previous version of this
> patch set with its latest PR.
>
> https://gitlab.com/qemu-project/qemu/-/commit/6005ee07c380cbde44292f5f6c96e7daa70f4f7d
>
> It is thus missing the v1->v2 changes. Basically some comments to
> clarify the optimization we're doing with the MR transaction and
> the removal of the optimization on an error path.
>
> The optimization on the error path isn't needed indeed but it
> doesn't hurt. No need to change that now that the patches are
> upstream.
>
> I can post a follow-up patch to add the missing comments though.
> While here, I'd even add these comments in the generic
> virtio_device_*_ioeventfd_impl() calls as well, since they already
> have the very same optimization.
>
> Anyway, I guess you can drop the patches from your tree.
Okay, I have dropped them.
Stefan
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- [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Greg Kurz, 2021/05/07
- [PATCH v2 1/4] virtio-blk: Fix rollback path in virtio_blk_data_plane_start(), Greg Kurz, 2021/05/07
- [PATCH v2 2/4] virtio-blk: Configure all host notifiers in a single MR transaction, Greg Kurz, 2021/05/07
- [PATCH v2 3/4] virtio-scsi: Set host notifiers and callbacks separately, Greg Kurz, 2021/05/07
- [PATCH v2 4/4] virtio-scsi: Configure all host notifiers in a single MR transaction, Greg Kurz, 2021/05/07
- Re: [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci, Stefan Hajnoczi, 2021/05/12