From: "John W. Linville" <linville@tuxdriver.com>
To: Nathaniel Smith <njs@pobox.com>
Cc: linux-wireless@vger.kernel.org,
bloat-devel@lists.bufferbloat.net, johannes@sipsolutions.net,
nbd@openwrt.org
Subject: Re: [RFC v2] mac80211: implement eBDP algorithm to fight bufferbloat
Date: Wed, 23 Feb 2011 17:28:43 -0500 [thread overview]
Message-ID: <20110223222842.GA20039@tuxdriver.com> (raw)
In-Reply-To: <AANLkTi=LnpFLzKbtaPxSjg9TRU_pY2oRi6tsFg5GkaGK@mail.gmail.com>
On Mon, Feb 21, 2011 at 03:26:32PM -0800, Nathaniel Smith wrote:
> On Mon, Feb 21, 2011 at 10:47 AM, John W. Linville
> <linville@tuxdriver.com> wrote:
> > I tried to see how your measurement would be useful, but I just don't
> > see how the number of frames ahead of me in the queue is relevant to
> > the measured link latency? I mean, I realize that having more packets
> > ahead of me in the queue is likely to increase the latency for this
> > frame, but I don't understand why I should use that information to
> > discount the measured latency...?
>
> It depends on which latency you want to measure. The way that I
> reasoned was, suppose that at some given time, the card is able to
> transmit 1 fragment every T nanoseconds. Then it can transmit n
> fragments in n*T nanoseconds, so if we want the queue depth to be 2
> ms, then we have
> n * T = 2 * NSEC_PER_MSEC
> n = 2 * NSEC_PER_MSEC / T
>
> Which is the calculation that you're doing:
>
> + sta->sdata->qdata[q].max_enqueued =
> + max_t(int, 2, 2 * NSEC_PER_MSEC / tserv_ns_avg);
>
> But for this calculation to make sense, we need T to be the time it
> takes the card to transmit 1 fragment. In your patch, you're not
> measuring that. You're measuring the total time between when a packet
> is enqueued and when it is transmitted; if there were K packets in the
> queue ahead of it, then this is the time to send *all* of them --
> you're measuring (K+1)*T. That's why in my patch, I recorded the
> current size of the queue when each packet is enqueued, so I could
> compute T = total_time / (K+1).
Thanks for the math! I think I see what you are saying now. Since the
measured time is being used to determine the queue size, we need to
factor-in the length of the queue that resulted in that measurment.
Unfortunately, I'm not sure how to apply this with the technique I
am using for the timing measurements. :-( I'll have to think about
this some more...
John
--
John W. Linville Someday the world will need a hero, and you
linville@tuxdriver.com might be all we have. Be ready.
next prev parent reply other threads:[~2011-02-23 22:30 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-02-13 17:56 [PATCH 0/5] iwlwifi: Auto-tune tx queue size to maintain latency under load Nathaniel J. Smith
2011-02-13 17:56 ` [PATCH 1/5] iwlwifi: Simplify tx queue management Nathaniel J. Smith
2011-02-14 9:57 ` Johannes Berg
2011-02-14 22:17 ` Nathaniel Smith
2011-02-14 22:45 ` wwguy
2011-02-15 0:15 ` Dave Täht
2011-02-16 9:16 ` Stanislaw Gruszka
2011-02-16 14:41 ` John W. Linville
2011-02-16 15:13 ` wwguy
2011-02-15 12:11 ` Johannes Berg
2011-02-14 15:33 ` wwguy
2011-02-13 17:56 ` [PATCH 2/5] iwlwifi: Convert the tx queue high_mark to an atomic_t Nathaniel J. Smith
2011-02-14 12:16 ` Johannes Berg
2011-02-14 22:35 ` Nathaniel Smith
2011-02-15 12:08 ` Johannes Berg
2011-02-15 17:37 ` Nathaniel Smith
2011-02-13 17:56 ` [PATCH 3/5] iwlwifi: Invert the sense of the queue high_mark Nathaniel J. Smith
2011-02-13 17:56 ` [PATCH 4/5] iwlwifi: auto-tune tx queue size to minimize latency Nathaniel J. Smith
2011-02-14 12:17 ` Johannes Berg
2011-02-14 21:58 ` Nathaniel Smith
2011-02-15 12:13 ` Johannes Berg
2011-02-15 15:03 ` John W. Linville
2011-02-16 8:59 ` Johannes Berg
2011-02-15 17:31 ` Nathaniel Smith
2011-02-14 15:46 ` wwguy
2011-02-13 17:56 ` [PATCH 5/5] iwlwifi: make current tx queue sizes visible in debugfs Nathaniel J. Smith
2011-02-14 0:32 ` [PATCH 0/5] iwlwifi: Auto-tune tx queue size to maintain latency under load Julian Calaby
2011-02-14 3:28 ` Nathaniel Smith
2011-02-16 15:50 ` John W. Linville
2011-02-16 23:08 ` Nathaniel Smith
2011-02-16 23:42 ` wwguy
2011-02-17 1:49 ` [RFC] mac80211: implement eBDP algorithm to fight bufferbloat John W. Linville
2011-02-17 3:31 ` Ben Greear
2011-02-17 4:26 ` Nathaniel Smith
2011-02-17 8:31 ` Johannes Berg
2011-02-18 21:21 ` [RFC v2] " John W. Linville
2011-02-19 3:44 ` Nathaniel Smith
2011-02-21 18:47 ` John W. Linville
2011-02-21 23:26 ` Nathaniel Smith
2011-02-23 22:28 ` John W. Linville [this message]
2011-02-25 18:21 ` Nathaniel Smith
2011-02-25 18:27 ` Nathaniel Smith
2011-02-20 0:37 ` Nathaniel Smith
2011-02-21 18:52 ` John W. Linville
2011-02-21 15:28 ` Johannes Berg
2011-02-21 19:06 ` John W. Linville
2011-02-21 20:26 ` Tianji Li
2011-02-28 13:07 ` Johannes Berg
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