On Tue, 2015-02-10 at 08:49 -0800, Jasper St. Pierre wrote:
Right now the way g_file_read_async works is by scheduling a task on a worker thread, having the worker thread do the async read, and then returning a result. As such, it's impossible to have two async reads done at the same time, which is really unfortunate from my understanding. If I'm reading a large file, and then a small file, the large file needs to fully complete before the small file is dispatched from the async queue.
From my reading of the GTask code, it should support 10 parallel reads (or other operations) since it uses a thread pool with maximum 10 threads. See g_task_thread_pool_init(). Is this not the case?
Additionally, when profiling GNOME on ARM, I've been seeing a lot of cases of users using g_file_read_async() "just in case" for no particular reason, which causes several locks to be taken, severely slowing performance.
I guess the particular reason depends on what’s being read, as discussed in the rest of the thread.
We need to seriously improve our async performance before asking people to use it.
That’s important, but I wouldn’t say it’s a blocker. The original idea of this thread was to try and address the situation where non-expert developers are using sync APIs when they should be using async ones, and then tying themselves in all sorts of knots. I suspect that hitting these sorts of performance problems is not going to be an issue for them — but freezing up the main loop is. I could be wrong, but that’s how I see it. Philip
On Tue, Feb 10, 2015 at 6:48 AM, Lennart Poettering
<mztabzr 0pointer de> wrote:
On Tue, 10.02.15 13:59, Philip Withnall
(philip tecnocode co uk) wrote:
> > I am pretty sure if you do async IO like gio does for
every single
> > file access you'll just complicate your program and make
it
> > substantially slower. For small files normal, synchronous
disk access
> > is a ton faster than dispatching things to background
threads, and
> > back...
>
> The problem is that GIO can’t know which accesses are to
small, local
> files, and which aren’t. It already optimises reads from
pollable
> streams (sockets) by keeping them in the main thread and
adding them
> into the main poll() call.
Well, but the developer frequently knows that. He knows that
the
config file in ~/.config is not going to be more than a few K.
And
that it hence is fine to access it synchronously...
> > Also, glib has wrappers for making mmaping available to
programs, to
> > improve seldom-accessed sparse databases efficient, do you
want to
> > prohibit that too?
>
> No, mmap() is clearly a tool for a different kind of
problem. If you’re
> accessing an mmap()ed file, you need to be sure it’s local
anyway, I
> think? GMappedFile doesn’t have async versions of its
methods,
> presumably for this reason.
mmap() works pretty Ok these days over NFS. Concurrent access
doesn't. But as long as you just want to access something,
it's
fine...
That said it's probably not a good idea to use mmap() for
stuff below
$HOME...
> As above, how about making that line the distinction between
calling
> functions from gstdio.h and using GIO? In the former case,
you know
> you’re operating on local files. In the latter, you could be
operating
> on files from the moon.
I'd always leave some freedom for the developers. It is
certainly good
to document things and push people into the right directions,
but I
think there are many cases where the developer should have
every right
to prefer sync access for valid reasons, even from the main
loop...
Lennart
--
Lennart Poettering, Red Hat
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Jasper
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