[Mesa-dev] [Bug 93686] Performance improvement ?=:=?UTF-8?Q? Please consider hardware ɢᴘᴜ rendering in llvmpipe

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=93686

--- Comment #28 from Jason Ekstrand <jason at jlekstrand.net> ---
(In reply to ytrezq from comment #15)
> (In reply to Alex Deucher from comment #13)
> > I can't speak for intel, but on AMD APUs, while the GPU appears as a device
> > on the PCIE bus, it actually has a much faster internal connection to the
> > memory controller.
> You’re still confusing things. Of course they use the same memory controller
> directly. Of course they share the same memory modules.
> However they can’t read or write in memory of each others. So this behave
> like an external card ᴘᴄɪe card with it’s own memory modules (if we forget
> the bandwidth is also shared with an another device so each ones slow each
> others).
> 
> So if you want to send or receive data it can only happens over the ᴘᴄɪe
> bus, triggering the same synchronisations problems of external chipsets due
> to the bus bandwidth and instructions overhead.
> 
> Unified memory will only happen in future generations of graphics cards, but
> only behind a ᴘᴄɪe bus (which will slow things because there’s still the
> memory controller bus adding overhead), so we’re far from the time were ɢᴘᴜs
> of ᴀᴘᴜs will be able to access ʀᴀᴍ of ᴄᴘᴜs with the same overhead (at that
> time it’s expected ʀᴀᴍ modules would have been merged into the ᴄᴘᴜ chip
> meaning there would be no longer separated ʀᴀᴍ modules).

This is just plain false.  Intel GPUs have, for a very long time shared memory
with the CPU.  They both have access to the exact same physical pages.  The
memory segregation you are referring to was a quirk of the Windows drivers of
the time.  The Linus driver, as far as I know, has always allowed the GPU and
CPU to freely access the same memory.  On big-core systems (i3, i5, i7), they
even share the same L3 cache so access is coherent between the two.

(In reply to ytrezq from comment #26)
> Concerning synchronisation, it don’t removes shared bandwidth with the ᴄᴘᴜ
> http://superuser.com/q/789816/282033

Please don't post links to random user focussed Q&A sites or forums to try and
explain to us how our hardware works.  The people you are talking to here are
driver developers!  We (AMD and Intel at least) have access to the real
documentation and know the hardware details very well.

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