On Dec 4, 2009, at 5:30 PM, Bryan O'Sullivan wrote:
On Fri, Dec 4, 2009 at 8:39 AM, Svein Ove Aas <[email protected]>
wrote:
That said.. you say you have to handle the events "fast". What happens
if you don't?
If you don't handle events quickly, they're typically thrown away by
the kernel without you ever getting to read them. That is, for
instance, what happens on Linux with inotify. Throwing away events
means that your app's internal mirror of the filesystem state
becomes wrong, which is Very Bad for most applications that care.
(i.e. Ross's assertion than nothing bad will happen is generally not
true.)
Ah hah yeah, I meant in the context of it won't block the kernel or
cause your computer to melt. It varies between applications whether
dropping events is bad or not so I wasn't commenting there.
However, with inotify you also can't afford to perform a single read
system call per event, because that will cause your "watch the
filesystem" event to soak up most of the system's CPU time. So what
you have to do is select to listen for "there's an event ready to be
read", then sleep a little while, then read in the hope that many
(but not too many!) events will have been queued that you can all
read at once.
And at that point, you'll be getting a stale notification about a
file or directory that may no longer even exist, or may have changed
type. Consider: I create a file f, write data into it, rename it to
g, then create a directory named f. You wake up 10 milliseconds
later, and the first event you hear about is that a file named f was
created.
This is all by way of saying that working with filesystem change
notification interfaces is extremely subtle and tricky, enormously
more so than you'd think on casual inspection. It's very easy to
write a program that uses these interfaces in ways that will make it
either generate garbage or consume huge amounts of CPU, and in fact
the common case is to write a program that does both.
Amen. I've written an application that does this kind of work using
inotify and it was a nightmare. I think this is why fseventsd was
invented for OS X, and I'm not sure if there's any linux equivalent.
However, if someone were to write a library that uses kqueue /
inotify / win32-call-I-forget-the-name-of-from-earlier-post in a way
that is both efficient and correct, that would be totally awesome.
-Ross
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