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Date: Sat, 19 Sep 2009 18:50:13 +0100
Message-ID: <6278d2220909191050l19f2a446r5ccf1907fd963096@mail.gmail.com>
Subject: Re: tickless and HZ=1000 throughput advantage?
From: Daniel J Blueman <daniel.blueman@gmail.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Linux Kernel <linux-kernel@vger.kernel.org>
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On Sat, Sep 19, 2009 at 3:55 PM, Arjan van de Ven <arjan@infradead.org> wrote:
> On Sat, 19 Sep 2009 15:47:24 +0100
> Daniel J Blueman <daniel.blueman@gmail.com> wrote:
>
>> On tickless kernels, is the general consensus that for non-embedded
>> systems, selecting HZ=1000 gives slightly more throughput in
>> particular situations than HZ=100 or 250, due to finer timer
>> intervals/granularity?
>
> it's not about throughput. It's about latency for some things....
> although now that select/poll and co use hrtimers it's not as critical
> anymore.
>
> the HZ timers aren't used much for anything time-critical nowadays.

Agreed. Do you think there is still a small case for moving to HZ=1000
(given it's effectively free) in situations like:

jiffies_to_transmit = port->baud?(1 + charsleft * 10 * HZ / port->baud):0;

<applying plausible figures>
(gdb) p (1 + 10 * 10 * 1000 / 38400) * 1
$3 = 3
(gdb) p (1 + 10 * 10 * 250 / 38400) * 4
$5 = 4

-> HZ=250 causes a 33% longer sleep than expected

perhaps?
-- 
Daniel J Blueman
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