On 28/09/15 21:21, Marek Olšák wrote:
On Mon, Sep 28, 2015 at 9:28 PM, Jose Fonseca <jfons...@vmware.com> wrote:
On 28/09/15 14:51, Marek Olšák wrote:
On Mon, Sep 28, 2015 at 2:55 PM, Jose Fonseca <jfons...@vmware.com> wrote:
On 27/09/15 19:14, Marek Olšák wrote:
Hi,
For some reason, st/mesa assumes that shaders can't be shared by
multiple contexts and therefore has a context pointer in shader keys.
I'd like to get rid of this dependency, because there is no reason why
shaders would need to be tied to a context. In fact, shaders don't
even need a device, they just need a compiler.
This is becoming a bigger issue with latest games that might prefer
compiling shaders in another thread using a different GL context.
As far as I know, shaders should be context-shareable on all hardware
drivers.
The thing to keep in mind is that, although in theory gallium shaders
could
be compiled once and reused by many pipe contexts, in practice many
drivers
need to produce shader variants that mix in state from other context
specific state objects, at draw time.
llvmpipe, svga pipe driver are examples of such drivers that I know.
Maybe
amd/nvidia gallium drivers are immune from that problem, but my
understanding was that very few hardware has internal state objects that
line up perfectly with gallium state objects, hence that this problem was
widespread.
And because until now shaders were per-context, this means that pipe
drivers
can attach the variants to the shader object without any worries about
multi-threading, as it's guaranteed that no other thread is operating on
them.
I'm well aware of that issue.
If you're aware but didn't think worth mention, then you clearly are
underestimating the issue.
Multi-threading is never easy. I think it's worth for you to open your mind
and think things through more thoroughly.
I thought about it very thoroughly and I'm quite sure about it. See below.
You can use a mutex to prevent 2 threads
from adding 2 shader variants simultaneously into a pipe shader. This
should prevent corrupting the linked list of variants. Looking up the
required shader variant based on states doesn't need locks if you add
an atomic counter containing how many shaders are in the list,
preventing the lookup function from having to read the ending pointer
of the list where new shaders can be added by other threads.
Drivers also shouldn't attach state pointers to shaders, but instead
they should add their copies there. All in all, this is quite easily
fixable and therefore not a big issue. It certainly doesn't prevent
shaders from becoming screen objects.
So, if you want to make pipe shaders global, you need to ensure that all
shaders objects manipulations are thread-safe. In particular, if you
still
want to attach variants to the shaders, you need to ensure access to
mutable
members are protected with mutex. Which means you might be forced to
lock
mutexes on hot-paths, severely impacting performance (single- and multi-
thread apps alike).
The mutex would only have to be locked when a new shader variant is
added, which is a time-consuming operation by itself (compilation
etc.), so there is no performance concern from adding a mutex.
Other cases don't need the lock.
I don't see how that can work.
You need to lock, not only when adding variants, but _all_ accesses,
including traversing the variants to search for the right one.
That is, _every_ time you need to search for a variant in the screen shader,
you'll need to lock the shader. If you don't, a different thread might
write to the variant list/hashtable exactly at the same time you're
traversing it, invalidating the pointers you're using to iterate over the
variants.
In other words, you'll be locking mutexes on pretty much every draw call...
Personally I think that's a poor design. I don't know how much is the mutex
lock overhead is in practice nowadays, but doing it per draw seems bad,
because it could all be avoided with a better design.
No, I don't need the lock while traversing the variants. I don't use a
hash table or a linked list from util. Our shader state has a pointer
to the first shader variant. The variant itself has a pointer to the
next variant. As long as I keep an atomic counter containing the
number of variants, I will only have to traverse that number of
variants. The variants itself are immutable. The list itself is
immutable except for the "next" pointer in the last variant, which I
will never access without the lock, because the atomic counter tells
me how many I have.
If the driver doesn't find the variant in the list, it will have to
acquire the lock, search the list again (see "double-checked locking
optimization"), if it still can't find it, then it will compile a new
variant and add it to the list. Then unlock.
This is fairly lockless except for the compilation and even during the
compilation, it remains lockless for contexts that only use
already-compiled variants.
However, I do understand that using a more complicated list or hash
table for the variants requires locking everywhere.
I see. Reads would be lock-free, but at the expense that variants could
only be added -- never removed --, right?
Maybe that's perfectly acceptable on most cases, but I wonder if it is
acceptable in all cases. Like embedded systems, or when the app is
running some untrusted code like WebGL implementations.
I still feel that a design that forces drivers down this very narrow
path is inferior.
One could avoid the threading primitives by attaching the variants to the
context themselves -- but then that's no different from what we do today
--
ie, you'd be compiling as often as today.
Like I said above, it's easy to add shader variants to screen-based
shaders with a mutex that has no impact on performance.
I disagree with your option 1). But if you're still determined, by all
means, do option 2). I think you'll regret it though: down the road you'll
be either struggling with multi-thread correctness or performance.
I don't feel too strongly either way: maybe the benefits of having
shaders
shared by many pipe context are indeed worth the trouble. But let it be
no
doubt: that implies making all operations on shader objects thread safe.
And that is not a trivial matter. It's certainly not as easy as just
coping
with a null pipe_context argument.
I see only 2 options out of this:
1) Removing the context pointer from the shader keys. If drivers need
this, they should handle it by themselves. This will simplify st/mesa,
because TCS, TES, GS won't need the variant-tracking machinery.
2) Adding PIPE_CAP_SHAREABLE_SHADERS and if the cap is 1, set the
context pointer to NULL in all keys.
What do you think?
Marek
I think there are other options worth considering:
a) I wonder if wouldn't be better the other way around: if drivers want
to
avoid compiling the same shader for many contexts, they should maintain a
shader cache internally that's shared by all contexts.
As that can be done much more efficient -- there are many ways to
implement a global cache that avoids frequent locking
And the way to avoid reimplementing the wheel on every driver would
be to
have an helper module that would take care of that -- ie, a generial
auxiliary module to maintain the shader cache in a thread safe maner.
That
module could potentially even learn to (de)serializing the cache from
disk.
This is too complicated for now and not required for having
screen-based shaders.
b) split pipe shader objects in two parts:
1) a global object, which would allow the pipe drivers to pre-compile
the
TGSI into an internal IR
2) per-context objects, so that pipe drivers could produce variants
of
that IR which takes in account the currently bound state
And for pipe drivers that can't pre-compile anything, 1) would be
nothing
more than a duplication of TGSI tokens.
I assume most drivers just don't want per-context shaders, so adding
an interface for it would be counterproductive.
Maybe I didn't explain. State tracker can't choose between 1) and 2) -- it
needs to create both.
That is,
- state tracker would create a screen-shader from TGSI tokens (one for the
whole share group)
- then create a context-shader from the screen-shader, one for each context
that shader is used by (so drivers can attach the variants to the
per-context shaders without locking)
It's probably better and easier to generate TGSI in glLinkProgram,
release the GLSL IR, and patch TGSI manually based on the shader key.
This would eliminate most of the compilation overhead in st/mesa and
it would be a good cleanup in general.
You're talking about the Mesa shader variants (1 GLSL -> N TGSI), while
I'm talking about the pipe shader variants (1 TGSI -> N HW-specific
machine code).
I already tried to explain this twice, yet you still make comments that
show my message didn't get across. I don't know how else to explain
this. Rob appears to have understood. If you were serious when you
asked what others thought could you please read what I proposed with an
open mind?
Jose
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