netdev - Re: [patch net-next] virtio_net: add support for Byte Queue Limits

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Message-ID: <ZkSwbaA74z1QwwJz@nanopsycho.orion>
Date: Wed, 15 May 2024 14:54:05 +0200
From: Jiri Pirko <jiri@...nulli.us>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, edumazet@...gle.com,
	kuba@...nel.org, pabeni@...hat.com, jasowang@...hat.com,
	xuanzhuo@...ux.alibaba.com, virtualization@...ts.linux.dev,
	ast@...nel.org, daniel@...earbox.net, hawk@...nel.org,
	john.fastabend@...il.com
Subject: Re: [patch net-next] virtio_net: add support for Byte Queue Limits

Wed, May 15, 2024 at 12:12:51PM CEST, jiri@...nulli.us wrote:
>Wed, May 15, 2024 at 10:20:04AM CEST, mst@...hat.com wrote:
>>On Wed, May 15, 2024 at 09:34:08AM +0200, Jiri Pirko wrote:
>>> Fri, May 10, 2024 at 01:27:08PM CEST, mst@...hat.com wrote:
>>> >On Fri, May 10, 2024 at 01:11:49PM +0200, Jiri Pirko wrote:
>>> >> Fri, May 10, 2024 at 12:52:52PM CEST, mst@...hat.com wrote:
>>> >> >On Fri, May 10, 2024 at 12:37:15PM +0200, Jiri Pirko wrote:
>>> >> >> Thu, May 09, 2024 at 04:28:12PM CEST, mst@...hat.com wrote:
>>> >> >> >On Thu, May 09, 2024 at 03:31:56PM +0200, Jiri Pirko wrote:
>>> >> >> >> Thu, May 09, 2024 at 02:41:39PM CEST, mst@...hat.com wrote:
>>> >> >> >> >On Thu, May 09, 2024 at 01:46:15PM +0200, Jiri Pirko wrote:
>>> >> >> >> >> From: Jiri Pirko <jiri@...dia.com>
>>> >> >> >> >> 
>>> >> >> >> >> Add support for Byte Queue Limits (BQL).
>>> >> >> >> >> 
>>> >> >> >> >> Signed-off-by: Jiri Pirko <jiri@...dia.com>
>>> >> >> >> >
>>> >> >> >> >Can we get more detail on the benefits you observe etc?
>>> >> >> >> >Thanks!
>>> >> >> >> 
>>> >> >> >> More info about the BQL in general is here:
>>> >> >> >> https://lwn.net/Articles/469652/
>>> >> >> >
>>> >> >> >I know about BQL in general. We discussed BQL for virtio in the past
>>> >> >> >mostly I got the feedback from net core maintainers that it likely won't
>>> >> >> >benefit virtio.
>>> >> >> 
>>> >> >> Do you have some link to that, or is it this thread:
>>> >> >> https://lore.kernel.org/netdev/21384cb5-99a6-7431-1039-b356521e1bc3@redhat.com/
>>> >> >
>>> >> >
>>> >> >A quick search on lore turned up this, for example:
>>> >> >https://lore.kernel.org/all/a11eee78-b2a1-3dbc-4821-b5f4bfaae819@gmail.com/
>>> >> 
>>> >> Says:
>>> >> "Note that NIC with many TX queues make BQL almost useless, only adding extra
>>> >>  overhead."
>>> >> 
>>> >> But virtio can have one tx queue, I guess that could be quite common
>>> >> configuration in lot of deployments.
>>> >
>>> >Not sure we should worry about performance for these though.
>>> >What I am saying is this should come with some benchmarking
>>> >results.
>>> 
>>> I did some measurements with VDPA, backed by ConnectX6dx NIC, single
>>> queue pair:
>>> 
>>> super_netperf 200 -H $ip -l 45 -t TCP_STREAM &
>>> nice -n 20 netperf -H $ip -l 10 -t TCP_RR
>>> 
>>> RR result with no bql:
>>> 29.95
>>> 32.74
>>> 28.77
>>> 
>>> RR result with bql:
>>> 222.98
>>> 159.81
>>> 197.88
>>> 
>>
>>Okay. And on the other hand, any measureable degradation with
>>multiqueue and when testing throughput?
>
>With multiqueue it depends if the flows hits the same queue or not. If
>they do, the same results will likely be shown.

RR 1q, w/o bql:
29.95
32.74
28.77

RR 1q, with bql:
222.98
159.81
197.88

RR 4q, w/o bql:
355.82
364.58
233.47

RR 4q, with bql:
371.19
255.93
337.77

So answer to your question is: "no measurable degradation with 4
queues".


>
>
>>
>>
>>> 
>>> >
>>> >
>>> >> 
>>> >> >
>>> >> >
>>> >> >
>>> >> >
>>> >> >> I don't see why virtio should be any different from other
>>> >> >> drivers/devices that benefit from bql. HOL blocking is the same here are
>>> >> >> everywhere.
>>> >> >> 
>>> >> >> >
>>> >> >> >So I'm asking, what kind of benefit do you observe?
>>> >> >> 
>>> >> >> I don't have measurements at hand, will attach them to v2.
>>> >> >> 
>>> >> >> Thanks!
>>> >> >> 
>>> >> >> >
>>> >> >> >-- 
>>> >> >> >MST
>>> >> >> >
>>> >> >
>>> >
>>