On 27/02/2017 10:06, Chris Wilson wrote:
On Mon, Feb 27, 2017 at 09:55:10AM +0000, Tvrtko Ursulin wrote:
On 22/02/2017 08:44, Chris Wilson wrote:
I also think that's an argument for improving the general cache rather
than arguing against using it.
Well I wasn't concerned about the cache per se, but about whether it
is completely appropriate (best choice) to use it in this particular
case.
Because as I said before, for 1920x1080x32 we are talking about a
16KiB extremely short lived temporary allocation, vs the similar
size for the sg radix cache. But radix cache sticks around the the
lifetime of obj->mm.pages and it wouldn't otherwise be there since
AFAICS in practice no one really touches frame buffers in a way to
trigger its creation.
Those amounts of memory are not a concern, but again, is the
simplification of the code worth the conceptual downsides mentioned
above? Even if we considered 4K frame buffers, when both allocations
go to ~64KiB, would that change anything? I am not sure, probably
not for me.
So I am still unsure that we should go with this change.
Again, the complaint you have here are general concerns about caching
the mapping. Avoiding using the cache instead of improving the cache
seems the wrong approach.
Depends what kind of improvments to the cache you have in mind. If you
are thinking about size then I disagree, I think it is efficient enough
already. But if you are thinking about the lifetime management then it
is obvious from all that I have written so far that I would agree with
that. Since the core of my "complaint" is the lifetime mismatch, and not
the size.
For lifetime I am not sure what you could do. Exposing the size of it,
with maybe some other bits attached the the object, to the shrinker I
think doesn't make much sense since the sizes are so small compared to
the backing store sizes.
Perhaps you could add an explicit reset of the cache after the rotation
is done with it, but then the only remaining benefit will be avoiding
greater than zero order allocations. I say the only one.. it would still
be a good one. Just have no idea if this level of cache usage would
satisfy you!
Perhaps you could say what kind of optimisation you have in mind to save
me guessing? :)
Regards,
Tvrtko
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