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From: | Denis Plotnikov |
Subject: | Re: [PATCH v1 2/4] virtio: increase virtuqueue size for virtio-scsi and virtio-blk |
Date: | Wed, 8 Sep 2021 18:20:49 +0300 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0 |
On 08.09.2021 16:22, Stefano Garzarella wrote:
Message bounced, I use new Denis's email address. On Wed, Sep 08, 2021 at 03:17:16PM +0200, Stefano Garzarella wrote:Hi Denis,I just found this discussion since we still have the following line in hw/core/machine.c:{ "vhost-blk-device", "seg_max_adjust", "off"}IIUC it was a typo, and I think we should fix it since in the future we can have `vhost-blk-device`.So, I think we have 2 options: 1. remove that line since for now is useless 2. replace with "vhost-scsi" I'm not sure which is the best, what do you suggest? Thanks, Stefano
Hi StefanoI prefer to just remove the line without replacing. This will keep things exactly like it is now.
Replacing with "vhost-scsi" will affect seg_max (limit to 126) on newly created VMs with machine types using "hw_compat_4_2" and older.
Now, because of the typo (error), their seg-max is queue-size dependent. I'm not sure, if replacing the line may cause any problems, for example on migration: source (queue-size 256, seg max 254) -> destination (queue-size 256, seg max 126). But it will definitely introduce two different behaviors for VMs with "hw_compat_4_2" and older. So, I'd choose the lesser of two evils and keep the things like it's now.
Thanks! Denis
On Fri, Feb 07, 2020 at 11:48:05AM +0300, Denis Plotnikov wrote:On 05.02.2020 14:19, Stefan Hajnoczi wrote:After thinking about that for a while, I think we shouldn't extend queue sizes for vhost-user-blk, vhost-user-scsi and vhost-scsi. This is because increasing the queue sizes seems to be just useless for them: the all thing is about increasing the queue sizes for increasing seg_max (it limits the max block query size from the guest). For virtio-blk-device and virtio-scsi-device it makes sense, since they have seg-max-adjust property which, if true, sets seg_max to virtqueue_size-2. vhost-scsi also have this property but it seems the property just doesn't affect anything (remove it?). Also vhost-user-blk, vhost-user-scsi and vhost-scsi don't do any seg_max settings. If I understand correctly, their backends are ment to be responsible for doing that. So, what about changing the queue sizes just for virtio-blk-device and virtio-scsi-device?On Tue, Feb 04, 2020 at 12:59:04PM +0300, Denis Plotnikov wrote:On 30.01.2020 17:58, Stefan Hajnoczi wrote:On Wed, Jan 29, 2020 at 05:07:00PM +0300, Denis Plotnikov wrote:vhost-blk-device?! Who has this? It's not in qemu.git so please omitThe goal is to reduce the amount of requests issued by a guest on1M reads/writes. This rises the performance up to 4% on that kind ofdisk access pattern. The maximum chunk size to be used for the guest disk accessing is limited with seg_max parameter, which represents the max amount of pices in the scatter-geather list in one guest disk request. Since seg_max is virqueue_size dependent, increasing the virtqueue size increases seg_max, which, in turn, increases the maximum size of data to be read/write from guest disk. More details in the original problem statment: https://lists.gnu.org/archive/html/qemu-devel/2017-12/msg03721.html Suggested-by: Denis V. Lunev <den@openvz.org> Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com> --- hw/core/machine.c | 3 +++ include/hw/virtio/virtio.h | 2 +- 2 files changed, 4 insertions(+), 1 deletion(-) diff --git a/hw/core/machine.c b/hw/core/machine.c index 3e288bfceb..8bc401d8b7 100644 --- a/hw/core/machine.c +++ b/hw/core/machine.c @@ -28,6 +28,9 @@ #include "hw/mem/nvdimm.h" GlobalProperty hw_compat_4_2[] = { + { "virtio-blk-device", "queue-size", "128"}, + { "virtio-scsi-device", "virtqueue_size", "128"}, + { "vhost-blk-device", "virtqueue_size", "128"},this line. ;-)So in this case the line: { "vhost-blk-device", "seg_max_adjust", "off"}, introduced by my patch: commit 1bf8a989a566b2ba41c197004ec2a02562a766a4 Author: Denis Plotnikov <dplotnikov@virtuozzo.com> Date: Fri Dec 20 17:09:04 2019 +0300 virtio: make seg_max virtqueue size dependent is also wrong. It should be: { "vhost-scsi-device", "seg_max_adjust", "off"}, Am I right?It's just called "vhost-scsi": include/hw/virtio/vhost-scsi.h:#define TYPE_VHOST_SCSI "vhost-scsi"On the other hand, do you want to do this for the vhost-user-blk,vhost-user-scsi, and vhost-scsi devices that exist in qemu.git? Thosedevices would benefit from better performance too.DenisIt seems to be so. We also have the test checking those settings: tests/acceptance/virtio_seg_max_adjust.py For now it checks virtio-scsi-pci and virtio-blk-pci. I'm going to extend it for the virtqueue size checking. If I change vhost-user-blk, vhost-user-scsi and vhost-scsi it's worthto check those devices too. But I don't know how to form a command line for that 3 devices since they should involve some third party components as backends (kernel modules, DPDK, etc.) and they seems to be not available inthe qemu git.Is there any way to do it with some qit.qemu available stubs or somethingelse? If so, could you please point out the proper way to do it?qemu.git has contrib/vhost-user-blk/ and contrib/vhost-user-scsi/ ifyou need to test those vhost-user devices without external dependencies.Stefan
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