On Thu, 13 Nov 2008, Tejun Heo wrote:

quoted text
> 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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