netdev - Re: [PATCH v4 0/10] bql: Byte Queue Limits

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Message-ID: <CAA93jw6bvzASrtvjhKL+haZb0VEPvQQD0guf7aBTPxJbxydEAg@mail.gmail.com>
Date:	Tue, 29 Nov 2011 09:51:43 +0100
From:	Dave Taht <dave.taht@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	John Fastabend <john.r.fastabend@...el.com>,
	Tom Herbert <therbert@...gle.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH v4 0/10] bql: Byte Queue Limits

On Tue, Nov 29, 2011 at 9:43 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mardi 29 novembre 2011 à 09:37 +0100, Dave Taht a écrit :
>> On Tue, Nov 29, 2011 at 8:23 AM, John Fastabend
>> <john.r.fastabend@...el.com> wrote:
>> >
>> > I wonder if we should consider enabling TSO/GSO per queue or per traffic
>> > class on devices that support this. At least in devices that support
>> > multiple traffic classes it seems to be a common usage case to put bulk
>> > storage traffic (iSCSI) on a traffic class and low latency traffic on a
>> > separate traffic class, VoIP for example.
>>
>>
>> VOIP is a drop in the bucket.
>>
>> Turning TSO off on TCP exiting the datacenter (or more specifically),
>> destined anywhere there is potential tx/rx bandwidth disparity
>> would be goooooood.
>>
>
> If your cpu is fast enough (and they are most of the time), this makes
> no difference at all.
>
> Instead of consuming 3% of cpu with TSO, you'll consume 10% or 15% and
> no difference seen on the wire.

Perhaps I don't understand the gross effects of TSO very well, but if you have
100 streams coming from a server, destined to X different destinations,
and you FQ to each on a per packet basis, you end up impacting the downstream
receive buffers throughout much less than if you send each stream as a burst.

>
> Really, if you want to avoid bursts, TSO has litle to do with them.

If I'm misunderstanding the downstream effects of TSO, I stand corrected.

>
>
>
>



-- 
Dave Täht
SKYPE: davetaht
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FR Tel: 0638645374
http://www.bufferbloat.net
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