linux-kernel - Re: [fuse-devel] [PATCHSET] FUSE: extend FUSE to support more operations

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Date:	Tue, 18 Nov 2008 19:30:19 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Miklos Szeredi <miklos@...redi.hu>
CC:	fuse-devel@...ts.sourceforge.net, greg@...ah.com,
	linux-kernel@...r.kernel.org
Subject: Re: [fuse-devel] [PATCHSET] FUSE: extend FUSE to support more	operations

Hello,

Miklos Szeredi wrote:
>> Hmm... I'm not sure I understand what you're saying but if you're
>> talking about optimizing async case where notification occurs while the
>> poller is sleeping, I don't think it really matters.  That could be
>> common but they're not performance sensitive path.  As select/poll users
>> become busy, ->poll operation becomes more and more synchronous.
> 
> If poll does have to sleep (however short), then it's alwasy more
> synchronous than necessary.  It introduces an extra latency of two
> context switches per poll.  Yes, this means the performance hit will
> decrease as the sleep lengthens, but short sleep != no sleep.

If we're talking about poll(2) and select(2), in many cases, they don't
sleep at all as the load nears its limit due to the cost for scanning
idle fds.  Those apps usually reach 100% before hitting their bandwidth
limits (no sleep at all) and until they hit the bandwidth limit only the
overhead of scanning idle fds decreases.

What I was trying to say was that for poll(2) and select(2), the
optimization would be a bit empty as they're not gonna help any with
increasing load.  Optimization would still be helpful but cost/benefit
doesn't seem to make much sense to me.

>> If the client is using better interface like epoll, sending revents via
>> notification could help a bit but again the problem is that the ->poll
>> vfs interface is not ready for that.  e.g. How do you tell whether
>> ->poll is for epoll polling after the notification or an asynchronous
>> poll(2) being called after a read(2) after the notification arrived.
>> There simply isn't enough information to determine when the cached
>> revents (no matter how short the period of caching is) can be used or
>> not.  One thing we can do is to invalidate the received revents value on
>> every file operation and then reverting to synchronous call only when
>> cached revents is not available, but I don't really see good
>> justifications for such over complexity.
> 
> It's an optimization, and not a very complex one at that.  But yes, we
> should leave that till later, when the simple interface prooved itself
> working.

Yeap, we can cache @revents per file and clear it on any operation on
the file but as you said let's leave it for the day when it actually
comes biting.

Thanks.

-- 
tejun
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