linux-kernel - Re: kvm aio wishlist

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Message-ID: <492C2D11.2030308@redhat.com>
Date:	Tue, 25 Nov 2008 18:51:29 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	suparna@...ibm.com, Zach Brown <zach.brown@...cle.com>,
	linux-aio@...ck.org, Jeff Moyer <jmoyer@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: kvm aio wishlist

Ingo Molnar wrote:
>   
>> Perhaps a variant of syslet, that is kernel-only, and does:
>>
>> - always allocate a new kernel stack at io_submit() time, but not a 
>>   new thread
>>     
>
> such a N:M threading design is a loss - sooner or later we arrive to a 
> point where people actually start using it and then we want to 
> load-balance and schedule these entities.
>   

It's only N:M as long as its nonblocking.  If it blocks it becomes 1:1 
again.  If it doesn't, it's probably faster to do things on the same 
cache as the caller.

> So i'd suggest the kthread based async engine i wrote for syslets. It 
> worked well and for kernel-only entities it schedules super-fast - it 
> can do up to 20 million events per second on a 16-way box i'm testing 
> on. The objections about syslets were not related to the scheduling of 
> it but were mostly about the userspace API/ABI: you dont have to use 
> that.

I'd love to have something :)

I guess any cache and latency considerations could be fixed if
- we schedule a syslet for the first time when the thread that launched 
it exits to userspace
- we queue it on the current cpu's runqueue

In that case, for the nonblocking case syslets and fibrils would have 
very similar performance.

-- 
error compiling committee.c: too many arguments to function

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