linux-kernel - Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

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Date:	Wed, 21 Feb 2007 17:04:45 -0800
From:	"Michael K. Edwards" <medwards.linux@...il.com>
To:	"Ingo Molnar" <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Arjan van de Ven" <arjan@...radead.org>,
	"Christoph Hellwig" <hch@...radead.org>,
	"Andrew Morton" <akpm@....com.au>,
	"Alan Cox" <alan@...rguk.ukuu.org.uk>,
	"Ulrich Drepper" <drepper@...hat.com>,
	"Zach Brown" <zach.brown@...cle.com>,
	"Evgeniy Polyakov" <johnpol@....mipt.ru>,
	"David S. Miller" <davem@...emloft.net>,
	"Suparna Bhattacharya" <suparna@...ibm.com>,
	"Davide Libenzi" <davidel@...ilserver.org>,
	"Jens Axboe" <jens.axboe@...cle.com>,
	"Thomas Gleixner" <tglx@...utronix.de>
Subject: Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3

On 2/21/07, Ingo Molnar <mingo@...e.hu> wrote:
> threadlets (and syslets) are parallel contexts and they behave so -
> queuing and execution semantics are then ontop of that, implemented
> either by glibc, or implemented by the application. There is no
> 'pipeline' of requests imposed - the structure of pending requests is
> totally free-form. For example in threadlet-test.c i've in essence
> implemented a 'set of requests' with the submission site only interested
> in whether all requests are done or not - but any stricter (or even
> looser) semantics and ordering can be used too.

In short, you have a dataflow model with infinite parallelism,
implemented using threads of control mapped willy-nilly onto the
underlying hardware.  This has not proven to be a successful model in
the past.

> in terms of AIO, the best queueing model is i think what the kernel uses
> internally: freely ordered, with barrier support. (That is equivalent to
> a "queue of sets", where the queue are the barriers, and the sets are
> the requests within barriers. If there is no barrier pending then
> there's just one large freely-ordered set of requests.)

That's a big part of why Linux scales poorly for workloads that
involve a large volume of in-flight I/O transactions.  Unless you
essentially lock one application thread to each CPU core, with a
complete understanding of its cache sharing and latency relationships
to all the other cores, and do your own userspace I/O scheduling and
dispatching state machine -- which is what all industrial-strength
databases and other sorts of transaction engines currently do -- you
get the same old best-effort context-thrashing scheduler we've had
since Solaris 2.0.

Let's do something genuinely better this time, OK?

Cheers,
- Michael
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