On Tue, 2015-02-10 at 10:30 -0800, Jasper St. Pierre wrote:
One quick example: calling g_file_read_async on a GResourceFile spawns a new thread and does a synchronous stream read from a block already in memory. It should just be a single g_bytes_ref, but we have three different classes created, a thread spawned, and a large amount of locks to do the equivalent of memcpy.
That’s not good at all, and seems like it should be easy to fix by overriding the read_async vfunc for GResourceFile. Is there a bug filed about it? Philip
On Tue, Feb 10, 2015 at 8:49 AM, Jasper St. Pierre
<jstpierre mecheye net> 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.
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.
We need to seriously improve our async performance before
asking people to use it.
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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Jasper
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