linux-kernel - Re: [patch 05/11] syslets: core code

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Date:	Thu, 15 Feb 2007 19:46:56 +0100
From:	bert hubert <bert.hubert@...herlabs.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <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>,
	Ulrich Drepper <drepper@...hat.com>,
	Zach Brown <zach.brown@...cle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Benjamin LaHaise <bcrl@...ck.org>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [patch 05/11] syslets: core code

On Thu, Feb 15, 2007 at 09:42:32AM -0800, Linus Torvalds wrote:

> We know one interface: the current aio_read() one. Nobody really _likes_ 
[...]

> Others? We don't know yet. And exposing complex interfaces that may not be 
> the right ones is much *worse* than exposing simple interfaces (that 
> _also_ may not be the right ones, of course - but simple and 

>From humble userland, here's two things I'd hope to be able to do, although
I admit my needs are rather specialist.

1) batch, and wait for, with proper error reporting:
	socket();
	[ setsockopt(); ]
	bind();
	connect();
	gettimeofday();  // doesn't *always* happen
	send();
	recv();
	gettimeofday(); // doesn't *always* happen

	I go through this sequence for each outgoing powerdns UDP query
	because I need a new random source port for each query, and I
	connect because I care about errrors. Linux does not give me random
	source ports for UDP sockets.

	When async, I can probably just drop the setsockopt (for
	nonblocking). I already batch the gettimeofday to 'once per epoll
	return', but quite often this is once per packet.

2) 	On the client facing side (port 53), I'd very much hope for a way to
	do 'recvv' on datagram sockets, so I can retrieve a whole bunch of
	UDP datagrams with only one kernel transition.

	This would mean that I batch up either 10 calls to recv(), or one
	'atom' of 10 recv's.

Both 1 and 2 are currently limiting factors when I enter the 100kqps domain
of name serving. This doesn't mean the rest of my code is as tight as it
could be, but I spend a significant portion of time in the kernel even at
moderate (10kqps effective) loads, even though I already use epoll. A busy
PowerDNS recursor typically spends 25% to 50% of its time on 'sy' load.

This might be due to my use of get/set/swap/makecontext though.

	Bert

-- 
http://www.PowerDNS.com      Open source, database driven DNS Software 
http://netherlabs.nl              Open and Closed source services
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