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

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Date:	Thu, 13 Nov 2008 15:48:12 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	tj@...nel.org
CC:	miklos@...redi.hu, fuse-devel@...ts.sourceforge.net,
	greg@...ah.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCHSET] FUSE: extend FUSE to support more operations

On Thu, 13 Nov 2008, Tejun Heo wrote:
> poll/select/epoll can poll on massive number of files.  I don't think
> it's wise to have that many outstanding requests.  FUSE currently uses
> linear list to match replies to requests and libfuse will consume one
> thread per each poll if implemented like other requests.  It can be made
> asynchronous from libfuse tho.
> 
> I kind of like the original implementation tho.  The f_ops->poll
> interface is designed to be used like ->poll returning events if
> available immediately and queue for later notification as necessary.
> Notification is asynchronous and can be spurious (this actually comes
> pretty handy for low level implementation).  When notified, upper layer
> queries the same way using ->poll.  This is quite convenient for low
> level implementation as the actual logic of poll can live in ->poll
> proper while notifications can be scattered around places where events
> can occur.

Yes, that kind of interface is nice for f_ops->poll, and for libfuse.

But for the kernel interface it's inefficient.  A wake up event is 3
context switches instead of one.  And that's inherent in the interface
itself for no good reason.

Also there's again the question of userspace filesystem messing with
the caller: your original implementation allows the userspace
filesystem to block f_ops->poll() forever, which really isn't what
poll/select is about.

So I'd still argue for the simple POLL-request/POLL-notify protocol on
the kernel API, and possibly have the async notification similar to
the kernel interface on the library API.

Implementation wise I don't care all that much, but I'd actually
prefer if it was implemented using the traditional request/reply thing
and optimized (possibly later) to find requests in a more efficient
way than searching the linear list, which would benefit not just poll
but all requests.

Miklos
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