linux-kernel - Re: kvm aio wishlist

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Message-ID: <20081125165613.GI22504@elte.hu>
Date:	Tue, 25 Nov 2008 17:56:13 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
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


* Avi Kivity <avi@...hat.com> wrote:

> 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.

yes. Hence given that fibrills have various tradeoffs, we should do 
the syslet thread pool. The code is there and it works :)

	Ingo
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