[Linaro-mm-sig] Memory Management Discussion

[Tom Cooksey]

> -----Original Message-----
> From: linaro-mm-sig-bounces at lists.linaro.org [mailto:linaro-mm-sig-
> bounces at lists.linaro.org] On Behalf Of Daniel Vetter
> Sent: 20 April 2011 07:56
> To: Arnd Bergmann
> Cc: linaro-mm-sig at lists.linaro.org
> Subject: Re: [Linaro-mm-sig] Memory Management Discussion
>
> On Wed, Apr 20, 2011 at 8:25 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> > On Wednesday 20 April 2011 03:52:53 Clark, Rob wrote:
> >> From DRI perspective.. I guess the global buffer name is restricted
> >> to a 4 byte integer, unless you change the DRI proto..
> >
> > I still like the idea of using file handles to pass the buffers
> > between kernel subsystems. Maybe there could be an ioctl to encapsulate
> > a buffer from DRI in a file so we can give it to another subsystem,
> > and/or an ioctl to register a buffer from a file handle with DRI.
>
> That's been the original design of gem, i.e. using fd handles (and
> perhaps even passing them around in unix domain sockets). There's one
> small problem with that approach: You're quickly running out of fds
> with the linux default limit of 1024. Hence the roll-your-own approach.

There's a big difference between GEM and what we're trying to do. GEM is
designed to manage _all_ buffers used by the graphics hardware. What I
believe we're trying to do is only provide a manager which allows buffers
to be shared between devices. Of all the buffers and textures a GPU needs
to access, only a tiny fraction of them need to be shared between devices
and userspace processes. How large that fraction is I don't know, it might
still be approaching the 1024 limit, but I doubt it...

So, the buffers we're interested in sharing between different processes
and devices are:

* Decoded video buffers (from both cameras & video decoders)
* Window back-buffers
* System-wide theme textures and font glyph caches

... Anyone know of other candidates?

I guess the bottleneck will probably be the window compositor, as it will
need to have a reference to all window back buffers in the system. So, do
any of the DRI folks have any idea how many windows (with back buffers) a
typical desktop session has? Do minimised windows have back buffers
allocated in X11?

Even if there were as many as 100 top-level windows (which seems excessive),
each triple-buffered, that's still pretty far from the 1024 limit?


Cheers,

Tom



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