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From: Tom Herbert <therbert@google.com>
To: Dave Taht <dave.taht@gmail.com>
Cc: bloat <bloat@lists.bufferbloat.net>
Subject: Re: [Bloat] some (very good) preliminary results from fiddling with byte queue limits on 100Mbit ethernet
Date: Sat, 19 Nov 2011 13:53:47 -0800	[thread overview]
Message-ID: <CA+mtBx9A4j8PX3nmrWMLjE2Z1in0-HUo9kDLnbzb6mrkSnnYRQ@mail.gmail.com> (raw)
In-Reply-To: <CAA93jw7_m7Vf8uVLP1oSGbiZ=uLZpC8jPsM7wkacMRArwN7G2A@mail.gmail.com>

Thanks for trying this out Dave!

> With byte queue limits at mtu*3 + the SFQ qdisc, latency under load
> can be hammered
>  down below 6ms when running at a 100Mbit line rate. No CBQ needed.
>
I'm hoping that we didn't have to set the BQL max_limit.  I would
guess that this might indicate some periodic spikes in interrupt
latency (BQL will increase limit aggressively in that case).  You
might want to try adjusting the hold_time to a lower value.  Also,
disabling TSO might lower the limit.

Without lowering the max_limit, what values so you see for limit and
inflight?  If you set min_limit to a really big number (effectively
turn of BQL), what does inflight grow to?

> Anyway, script could use improvement, and I'm busily patching BQL into
> the ag71xx driver as I write.
>
Cool, I look forward to those results!

> Sorry it's taken me so long to get to this since your bufferbloat
> talks at linux plumbers. APPLAUSE.
> It's looking like BQL + SFQ is an effective means of improving
> fairness and reducing latency on drivers
> that can support it. Even if they have large tx rings that the
> hardware demands.
>
Great.  I actually got back to looking at this a little last week.
AFAICT the overhead of BQL is < 1% CPU and throughput (still need more
testing to verify that).  There are some (very) minor performance
improvements that might be possible, but I don't have any major
modifications pending at this point.

  reply	other threads:[~2011-11-19 21:53 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2011-11-19 20:33 Dave Taht
2011-11-19 21:53 ` Tom Herbert [this message]
2011-11-19 22:47   ` Dave Taht
2011-11-21 15:08 ` John W. Linville
2011-11-22  5:36   ` Simon Barber

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