[Mesa-dev] [PATCH] ra: Disable round-robin strategy for optimistically colorable nodes.
Francisco Jerez
currojerez at riseup.net
Mon Feb 16 10:40:52 PST 2015
Jason Ekstrand <jason at jlekstrand.net> writes:
> On Feb 16, 2015 9:34 AM, "Francisco Jerez" <currojerez at riseup.net> wrote:
>>
>> Jason Ekstrand <jason at jlekstrand.net> writes:
>>
>> > On Feb 16, 2015 8:35 AM, "Francisco Jerez" <currojerez at riseup.net>
> wrote:
>> >>
>> >> The round-robin allocation strategy is expected to decrease the amount
>> >> of false dependencies created by the register allocator and give the
>> >> post-RA scheduling pass more freedom to move instructions around. On
>> >> the other hand it has the disadvantage of increasing fragmentation and
>> >> decreasing the number of equally-colored nearby nodes, what increases
>> >> the likelihood of failure in presence of optimistically colorable
>> >> nodes.
>> >>
>> >> This patch disables the round-robin strategy for optimistically
>> >> colorable nodes. These typically arise in situations of high register
>> >> pressure or for registers with large live intervals, in both cases the
>> >> task of the instruction scheduler shouldn't be constrained excessively
>> >> by the dense packing of those nodes, and a spill (or on Intel hardware
>> >> a fall-back to SIMD8 mode) is invariably worse than a slightly less
>> >> optimal scheduling.
>> >
>> > Actually, that's not true. Matt was doing some experiments recently
> with a
>> > noise shader from synmark and the difference between our 2nd and 3rd
> choice
>> > schedulers is huge. In that test he disabled the third choice scheduler
>> > and the result was a shader that spilled 6 or 8 times but ran something
>> > like 30% faster. We really need to do some more experimentation with
>> > scheduling and figure out better heuristics than "SIMD16 is always
> faster"
>> > and "spilling is bad".
>> >
>>
>> Yes, I'm aware of rare corner cases like that where e.g. SIMD16 leads to
>> higher cache thrashing than SIMD8 leading to decreased overall
>> performance, and a case where a shader SIMD16 *with* spills has better
>> performance than the SIMD8 version of the same shader without spills.
>>
>> In any case it's not the register allocator's business to implement such
>> heuristics, and that's not an argument against the register allocator
>> trying to make a more efficient use of the register file.
>
> The primary point I was trying to make is that scheduling *does* matter.
> It matters a lot. In fact, Matt and i have talked about throwing away the
> SIMD16 program if it ends up using the pessimal schedulong algorithm.
> Throwing scheduling to the wind just to gain a few SIMD16 programs is
> probably not a good trade-off.
>
In my experience the exact opposite observation has been far more
common. Running SIMD16 vs SIMD8 has a larger impact on performance than
the way you end up scheduling things post-regalloc. Actually even if
you end up causing some unmet instruction dependencies by the way
instructions are scheduled post-regalloc, the EU can context-switch to
service the next available thread almost for free when a thread stalls
on some dependency. Also the fact that you're doing SIMD16 itself makes
post-regalloc scheduling less important because it naturally has an
effect in hiding latency.
My intuition is that the huge performance improvement Matt observed by
disabling the third scheduling heuristic is more likely to have been
caused by a decrease in the amount of cache thrashing caused by the fact
that he was running less channels concurrently rather than by the
scheduling heuristic itself. Matt, did you rule out that possibility?
The other thing is this patch has an effect on the allocation strategy
for optimistically colorable nodes *only*. We're already heavily
constrained by register pressure when we get to that point, and assuming
allocation succeeds the post-regalloc scheduler is going to have little
room for maneuvering anyway.
> It could be that this is an good idea, but it's going to take more than
> hand-waved theories about register allocation one shader not spilling to
> convince me. Do you actually know what it did to scheduling? It wouldn't
> be hard to hack up the driver and shader-db to collect that information.
>
44 shaders going SIMD16 seems like a strong enough argument to me.
Could you be more precise about what additional information you want me
to collect?
> --Jason
>
>> >> Shader-db results on the i965 driver:
>> >>
>> >> total instructions in shared programs: 5488539 -> 5488489 (-0.00%)
>> >> instructions in affected programs: 1121 -> 1071 (-4.46%)
>> >> helped: 1
>> >> HURT: 0
>> >> GAINED: 49
>> >> LOST: 5
>> >> ---
>> >> src/util/register_allocate.c | 22 +++++++++++++++++++++-
>> >> 1 file changed, 21 insertions(+), 1 deletion(-)
>> >>
>> >> diff --git a/src/util/register_allocate.c
> b/src/util/register_allocate.c
>> >> index af7a20c..d63d8eb 100644
>> >> --- a/src/util/register_allocate.c
>> >> +++ b/src/util/register_allocate.c
>> >> @@ -168,6 +168,12 @@ struct ra_graph {
>> >>
>> >> unsigned int *stack;
>> >> unsigned int stack_count;
>> >> +
>> >> + /**
>> >> + * Tracks the start of the set of optimistically-colored registers
> in
>> > the
>> >> + * stack.
>> >> + */
>> >> + unsigned int stack_optimistic_start;
>> >> };
>> >>
>> >> /**
>> >> @@ -454,6 +460,7 @@ static void
>> >> ra_simplify(struct ra_graph *g)
>> >> {
>> >> bool progress = true;
>> >> + unsigned int stack_optimistic_start = ~0;
>> >> int i;
>> >>
>> >> while (progress) {
>> >> @@ -483,12 +490,16 @@ ra_simplify(struct ra_graph *g)
>> >>
>> >> if (!progress && best_optimistic_node != ~0U) {
>> >> decrement_q(g, best_optimistic_node);
>> >> + stack_optimistic_start =
>> >> + MIN2(stack_optimistic_start, g->stack_count);
>> >> g->stack[g->stack_count] = best_optimistic_node;
>> >> g->stack_count++;
>> >> g->nodes[best_optimistic_node].in_stack = true;
>> >> progress = true;
>> >> }
>> >> }
>> >> +
>> >> + g->stack_optimistic_start = stack_optimistic_start;
>> >> }
>> >>
>> >> /**
>> >> @@ -542,7 +553,16 @@ ra_select(struct ra_graph *g)
>> >> g->nodes[n].reg = r;
>> >> g->stack_count--;
>> >>
>> >> - if (g->regs->round_robin)
>> >> + /* Rotate the starting point except for optimistically colorable
>> > nodes.
>> >> + * The likelihood that we will succeed at allocating
> optimistically
>> >> + * colorable nodes is highly dependent on the way that the
> previous
>> >> + * nodes popped off the stack are laid out. The round-robin
>> > strategy
>> >> + * increases the fragmentation of the register file and
> decreases
>> > the
>> >> + * number of nearby nodes assigned to the same color, what
>> > increases the
>> >> + * likelihood of spilling with respect to the dense packing
>> > strategy.
>> >> + */
>> >> + if (g->regs->round_robin &&
>> >> + g->stack_count <= g->stack_optimistic_start)
>> >> start_search_reg = r + 1;
>> >> }
>> >>
>> >> --
>> >> 2.1.3
>> >>
>> >> _______________________________________________
>> >> mesa-dev mailing list
>> >> mesa-dev at lists.freedesktop.org
>> >> http://lists.freedesktop.org/mailman/listinfo/mesa-dev
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