linux-kernel - Re: [take25 1/6] kevent: Description.

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Message-ID: <4566AE48.70409@cosmosbay.com>
Date:	Fri, 24 Nov 2006 09:33:12 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andrew Morton <akpm@...l.org>
CC:	Ulrich Drepper <drepper@...hat.com>, Jeff Garzik <jeff@...zik.org>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Zach Brown <zach.brown@...cle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Chase Venters <chase.venters@...entec.com>,
	Johann Borck <johann.borck@...sedata.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [take25 1/6] kevent: Description.

Andrew Morton a écrit :
> On Fri, 24 Nov 2006 01:48:32 +0100
> Eric Dumazet <dada1@...mosbay.com> wrote:
> 
>>> The alternative is the sorry state we have now.  In nscd, for instance, 
>>> we have one single thread waiting for incoming connections and it then 
>>> has to wake up a worker thread to handle the processing.  This is done 
>>> because we cannot "park" all threads in the accept() call since when a 
>>> new connection is announced _all_ the threads are woken.  With the new 
>>> event handling this wouldn't be the case, one thread only is woken and 
>>> we don't have to wake worker threads.  All threads can be worker threads.
>> Having one specialized thread handling the distribution of work to worker 
>> threads is better most of the time.
> 
> It might be now.  Think "commodity 128-way".  Your single distribution thread
> will run out of steam.
> 
> What Ulrich is proposing is faster.  This is a new interface.  Let's design
> it to be fast.

Hum... I guess you didnt read my mail... I basically agree with Ulrich.

I just wanted to say that a fast application cannot rely only on a "let's park 
N threads waiting for single event in this queue", and hope kernel will be 
smart for us.

Even with 128-ways, you still hit a central point of coordination (it can be a 
mutex in kevent code, a atomic uidx in userland, or whatever) for a 'kevent 
queue'. Once you paid the cache lines ping/pong, you wont be *fast*.

I wish *you* dont think of kevent of only dispatching HTTP 1.0 trivial web 
requests.

Being able to direct a particular request on a particular CPU is certainly 
something that cannot be hardcoded in 'the new kevent interface'.

Eric
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