linux-kernel - Re: threadlets as 'naive pool of threads', epoll, some measurements

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Date:	Mon, 26 Feb 2007 13:50:54 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Linux Kernel Mailing List <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>,
	"David S. Miller" <davem@...emloft.net>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: threadlets as 'naive pool of threads', epoll, some measurements


* Evgeniy Polyakov <johnpol@....mipt.ru> wrote:

> > yet another performance update - with the fixed 'heaps of stupid 
> > threads' evserver_threadlet.c code attached below i got:
> > 
> > >    evserver_epoll:             9400 reqs/sec
> > >    evserver_epoll_threadlet:   9400 reqs/sec
> > 
> >      evserver_threadlet:         9000 reqs/sec
> > 
> > so the overhead, instead of the 10x slowdown Evgeniy 
> > predicted/feared, is 4% for this particular, very event-centric 
> > workload.
> > 
> > why? because Evgeniy still overlooks what i've mentioned so many 
> > times: that there is lots of inherent 'caching' possible even in 
> > this particular '8000 clients' workload, which even the most stupid 
> > threadlet queueing model is able to take advantage of. The maximum 
> > level of parallelism that i've measured during this test was 161 
> > threads.
> 
> :)
> 
> I feared _ONLY_ situation when thousands of thereads are eating my 
> brain - so case when 161 threads are running simultanesoulsy is not 
> that bad compared to what micro-design can do (of its best/worst) at 
> all!

even with ten thousand threads it is still pretty fast. Certainly not 
'10 times slower' as you claimed. And it takes only a single, trivial 
outer event loop to lift it up to the performance levels of a pure event 
based server.

conclusion: currently i dont see a compelling need for the kevents 
subsystem. epoll is a pretty nice API and it covers most of the event 
sources and nicely builds upon our existing poll() infrastructure.

furthermore, i very much contest your claim that a high-performance, 
highly scalable webserver needs a kevent+nonblock design. Even if i 
ignore all the obvious usability and maintainance-cost advantages of 
threadlets.

> So, caching is good - threadlets do not spawn a new thread, kevent 
> returns immediately, but in case of things are not that shine - 
> threadlets spawnd a new thread, while kevent process next request or 
> waits for all completed.

no. Please read the evserver_threadlet.c code. There's no kevent in 
there. There's no epoll() in there. All that you can see there is the 
natural behavior of pure threadlets. And it's not a workload /I/ picked 
for threadlets - it is a workload, filesize, parallelism level and 
request handling function /you/ picked for "event-servers".

	Ingo
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