linux-kernel - Re: [RFC][PATCH] create workqueue threads only when needed

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Date:	Mon, 02 Feb 2009 09:42:45 +0100
From:	Stefan Richter <stefanr@...6.in-berlin.de>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC][PATCH] create workqueue threads only when needed

Benjamin Herrenschmidt wrote:
> On Mon, 2009-02-02 at 03:24 +0100, Frederic Weisbecker wrote:
>> On Mon, Feb 02, 2009 at 08:37:41AM +1100, Benjamin Herrenschmidt wrote:
>> > 
>> > > I don't know, most of those I've looked on are not documented about the reason
>> > > for a private workqueue. I guess most of them can use the usual kevent.

I rather suspect that the majority of private workqueues are there for
good reasons.

>> > The main problem with kevent is that it gets clogged up.
>> 
>> I don't think so. Here is a snapshot of the workqueue tracer in my
>> box currently:
> 
> That's not quite what I meant ... 
> 
> The main problem with keventd I'd say is that it's used in all sort of
> exeptional code path (ie, driver reset path, error handling, etc...) for
> things that will msleep happily for tenth milliseconds, that sort of
> thing.
> 
> IE. It will be pretty responsive -in general- but can suffer form
> horrible latencies every now and then.

Actually it /should/ be the other way around:

The shared workqueue should only be used for work that sleeps only
briefly (perhaps with the exception of very unlikely longer sleeps e.g.
for allocations that cause paging).

Work which /may/ sleep longer, for example performs SCSI transactions,
needs to go into a private workqueue or other kind of context.

OTOH you are right too; work which must not be deferred too long by work
from another uncooperative/ unfair subsystem is probably also better off
in an own workqueue...
-- 
Stefan Richter
-=====-==--= --=- ---=-
http://arcgraph.de/sr/
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