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

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Date:	Tue, 27 Feb 2007 13:34:21 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Theodore Tso <tytso@....edu>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ulrich Drepper <drepper@...hat.com>,
	linux-kernel@...r.kernel.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>,
	Zach Brown <zach.brown@...cle.com>,
	"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

* Theodore Tso <tytso@....edu> wrote:

> I think what you are not hearing, and what everyone else is saying 
> (INCLUDING Linus), is that for most programmers, state machines are 
> much, much harder to program, understand, and debug compared to 
> multi-threaded code. [...]

btw., another crutial thing that i think Evgeniy is missing is that 
threadlets /enable/ event loops to be used in practice! Right now the 
epoll/kevent programming model requires a total 100% avoidance of all 
context-switching in the 'main' event handler context while handling a 
request. If just 1% of all requests happen to block it might cause a 
/complete/ breakdown of an event loop's performance - it can easily 
cause a 10x drop in performance or worse!

So context-switching has to be avoided in 100% of the code that runs 
while handling requests, file descriptors have to be set to nonblocking 
(causing extra system calls), and all the syscalls that might return 
incomplete with either -EINVAL or with a short read/write have to be 
converted into a state machine. (or in the alternative, user-space 
threading has to be used, which opens up another hornet's nest)

/That/ is the main inhibiting factor of the measured use of event loops 
within Linux! It has zero integration capabilities with 'usual' coding 
techniques - driving the costs of its application up in the sky, and 
pushing event based servers into niches.

With threadlets the picture changes dramatically: all we have to 
concentrate on to get the performance of "100% event based servers" is 
to handle 'most' rescheduling events in the event loop. A 10-20% context 
switching ratio does not hurt at all. (it causes ~1% of throughput 
loss.)

Furthermore, even if a particular configuration or module of the server 
(say Apache) happens to trigger a high rate of scheduling, the 
performance breakdown model of threadlets is /vastly/ superior to event 
based servers. The measurements so far have shown that the absolute 
worst-case threading server performance is at around 60% of that of 
non-context-switching servers - and even that level is reached 
gradually, leaving time for action for the server owner. While with 
fully event based servers there are mostly only two modes of 
performance: 100% performance and near-0% performance: total breakdown.

	Ingo
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