[llvm-dev] [llvm-mca] Resource consumption of ProcResGroups

Sun May 10 09:59:42 PDT 2020

Hi Alex,

On Sun, May 10, 2020 at 4:00 PM Alex Renda <renda at csail.mit.edu> wrote:

> Thanks, that’s very helpful!
>
>
>
> Also, sorry for the miscue on that bug with the 2/4 cycles — I realize now
> that that’s an artifact of a change that I made to not crash when resource
> groups overlap without all atomic subunits being specified:
>
> `echo 'fxrstor (%rsp)' | llvm-mca -mtriple=x86_64-unknown-unknown
> -march=x86-64 -mcpu=haswell`
> crashes (because fxrstor requests
> `HWPort0,HWPort6,HWPort23,HWPort05,HWPort06,HWPort15,HWPort0156`, so
> HWPort0156 ends up asserting because 0,1,5, and 6 are all taken), so I
> added:
> ```
> --- a/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp
> +++ b/llvm/lib/MCA/HardwareUnits/ResourceManager.cpp
> @@ -292,7 +292,7 @@ void ResourceManager::issueInstruction(
> ResourceState &RS = *Resources[Index];
>
> - if (!R.second.isReserved()) {
> + if (!R.second.isReserved() && RS.isReady()) {
> ResourceRef Pipe = selectPipe(R.first);
> use(Pipe);
> BusyResources[Pipe] += CS.size();
> ```
> which is probably the cause of that weird behavior I reported.
>
>
>
> I’m also somewhat curious about what “NumUnits” is modeling: I haven’t
> totally worked through the code yet, but it seems that when more (not
> necessarily atomic) sub-resources of a resource group are requested, more
> “NumUnits” of that group is requested. This doesn’t seem particularly
> intuitive to me, at least in my mental model of Haswell scheduling (and
> also leads to some infinite loops like `echo ‘fldenv (%rsp)' | llvm-mca
> -mtriple=x86_64-unknown-unknown -march=x86-64 -mcpu=haswell`, which
> requests HWPort0,HWPort1,HWPort01,HWPort05, and HWPort015, meaning that
> HWPort015 never schedules because it requests 4 “NumUnits” but only 3 are
> ever available). Is there some particular behavior that this is modeling?
>

The issue with fxrstor is unfortunately a bug. I'll see if I can fix it.

Strictly speaking, that "NumUnits" quantity is literally the number of
processor resource units consumed. By construction, it should never exceed
the actual number of resource units declared by a processor resource in the
scheduling model (i.e. this particular field of MCProcResourceDesc -
https://llvm.org/doxygen/structllvm_1_1MCProcResourceDesc.html#a9d4d0cc34fcce4779dc4445d8265fffc
).

The reason why you see the crash/assert is because the number of resource
units for scheduler resources is incorrectly set for that instruction by
the instruction builder in llvm-mca ( lib/MCA/InstrBuilder.cpp  - function
`initializeUsedResources()`). I am looking at it.

About what Andy suggested: I really like the idea of having a vector of
delay-cycles for consumed resources. It would fix most of these problems in
Haswell, and it could be used to improve the description of some
instructions on other processor models too. I agree that it should not be
difficult to implement (at least, the tablegen part should be pretty
mechanical); the resource allocation logic in class ResourceManager (in
llvm-mca) would require a bit of refactoring. Other than that, the rest
should be doable.

-Andrea

>
> -Alex
> On May 10, 2020, 9:32 AM -0400, Andrew Trick <atrick at apple.com>, wrote:
>
>
>
> On May 9, 2020, at 5:12 PM, Andrea Di Biagio via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> The llvm scheduling model is quite simple and doesn't allow mca to
> accurately simulate the execution of individual uOPs. That limitation is
> sort-of acceptable if you consider how the scheduling model framework was
> originally designed with a different goal in mind (i.e. machine
> scheduling). The lack of expressiveness of the llvm scheduling model
> unfortunately limits the accuracy of llvm-mca: we know the number of uOPs
> of an instruction. However we don't know which resources are consumed by
> which micro-opcodes. So we cannot accurately simulate the independent
> execution of individual opcodes of an instruction.
>
> Another "problem" is that it is not possible to describe when uOPs
> effectively start consuming resources. At the moment, the expectation is
> that resource consumption always starts at relative cycle #0 (relative to
> the instruction issue cycle).
> Example: an horizontal add on x86 is usually decoded into a pair of
> shuffles uOPs and a single (data-dependent) vector ADD uOP.
> The ADD uOP doesn't execute immediately because it needs to wait for the
> other two shuffle uOPs. It means that the ALU pipe is still available at
> relative cycle #0 and it is only consumed starting from relative cycle #1
> (ssuming that both shuffles can start execution at relative cycle #0). In
> practice, the llvm scheduling model only allows us to declare which
> pipeline resources are consumed, and for how long (in number cycles). So we
> cannot accurately describe to mca that the delayed consumption of the ALU
> pipe.
> Now think about what happens if: the first shuffle uOP consumes 1cy of
> HWPort0, and the second shuffle uOp consumes 1cy of HWPort1, and the ADD
> consumes 1cy of HWPort01. We end up in that "odd" situation you described
> where HWPort01 is "reserved" for 1cy.
> In reality, that 1cy of HWPort01 should have started 1cy after the other
> two opcodes. At that point, both pipelines would have been seen available.
>
> In conclusion, the presence of a "reserved" flag is not ideal, but it is
> sort-of a consequence of the above mentioned two limitations (plus the way
> how the Haswell and Broadwell models were originally designed).
>
> I hope it helps,
> -Andrea
>
>
> Food for thought...
>
> It would be easy to add a DelayCycles vector to SchedWriteRes to indicate
> the relative start cycle for each reserved resource. That would effectively
> model dependent uOps.
>
> NumMicroOps is only meant to model any general limitation of the cpu
> frontend to issue/rename/retire micro-ops. So, yes, there's no way to
> associate resources with specific uOps. You can mark any kind of resource
> as "dynamically scheduled" (BufferSize = -1). If an instruction uses
> different kinds of dynamic resources, then those need not be reserved at
> the same time. If we had the DelayCycles vector, it could be interpreted as
> "this resource must be reserved N cycles after prior reservations of other
> resources".
>
> -Andy
>
>
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