On Sat, 25 May 2002, Frank C. Earl wrote:
>
> Linus, if you're still listening in, can you spare us a moment to tell us
> what consequences quickly mapping and unmapping memory reigons into userspace
> has on the system?
It's reasonably fine on UP, and it often _really_ sucks on SMP.
On UP, the real cost is not so much the actual TLB invalidate (which works
at a page granularity anyway on any recent CPU), but the fact that you
need to walk the page tables (cache miss heaven), and you will eventually
need to fault another page back in (page fault, cache miss, whatever).
On SMP, especially if the program is threaded (which games often are:
even if the actual graphics engine is single-threaded, you end up having
another thread for sound, one possible for AI or input etc), the cost goes
up noticeably thanks to a (synchronous) CPU cross-call for a proper TLB
invalidate.
> We've got a couple of the DRM modules that do that to
> ensure the driver is secure. I'm thinking it's a source of some performance
> degredation in the drivers and it may not be good on the memory subsytem.
My gut feel is that especially under SMP, you're actually better off
copying stuff, especially if we're talking about buffers that are "mostly"
less than few kB.
Basically, if the data can fit in the cache (ie the app has just generated
them, and the data is already in the CPU cache and not big enough to blow
that cache to kingdom come), copying is almost guaranteed to be a win,
even on UP.
(And please do note the cache issues: while a big buffer can often improve
performance, it can equally easily _decrease_ performance by putting more
cache pressure on the system. You're often better off re-using a smaller
8kB buffer many times - and doing most everything out of the cache - than
trying to use a 1MB buffer and aiming for "perfect scaling").
DMA'ing directly from user space is most likely advantageous for doing
things like textures, which are bound to be fairly big anyway. I'd _hope_
that those don't have security issues (ie they'd be DMA'able as just data,
no command interface), but I don't have any information about the card
details.
Linus
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