From: Stefan Hajnoczi <stefanha@redhat.com>
To: Greg Kurz <groug@kaod.org>
Cc: Fam Zheng <fam@euphon.net>, Kevin Wolf <kwolf@redhat.com>,
qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>,
qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
Paolo Bonzini <pbonzini@redhat.com>,
David Gibson <david@gibson.dropbear.id.au>
Subject: Re: [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci
Date: Wed, 12 May 2021 17:05:53 +0100 [thread overview]
Message-ID: <YJv84RIViv6KvCHb@stefanha-x1.localdomain> (raw)
In-Reply-To: <20210507165905.748196-1-groug@kaod.org>
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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
Stefan
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next prev parent reply other threads:[~2021-05-12 16:10 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-07 16:59 [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci Greg Kurz
2021-05-07 16:59 ` [PATCH v2 1/4] virtio-blk: Fix rollback path in virtio_blk_data_plane_start() Greg Kurz
2021-05-07 16:59 ` [PATCH v2 2/4] virtio-blk: Configure all host notifiers in a single MR transaction Greg Kurz
2021-05-07 16:59 ` [PATCH v2 3/4] virtio-scsi: Set host notifiers and callbacks separately Greg Kurz
2021-05-07 16:59 ` [PATCH v2 4/4] virtio-scsi: Configure all host notifiers in a single MR transaction Greg Kurz
2021-05-12 16:05 ` Stefan Hajnoczi [this message]
2021-05-17 8:32 ` [PATCH v2 0/4] virtio: Improve boot time of virtio-scsi-pci and virtio-blk-pci Greg Kurz
2021-05-17 13:37 ` Stefan Hajnoczi
2021-05-17 23:33 ` Michael S. Tsirkin
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