Tipical cache and branch misses for a chess engine

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nionita
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Tipical cache and branch misses for a chess engine

Post by nionita » Wed Sep 14, 2016 5:09 pm

Because Barbarossa is written in Haskell, which allocates a lot of bytes all the time and then lets a garbage collecton run pretty often, I always assumed the cache misses must be very high in my engine. Also, because there is no way in Haskel to give the compiler a hint which part of a branch is expected to be more probable, I assumed that branch misses must be very high too.

But we know that our beliefs are mostly wrong and we better measure as much as possible, so this is what I did, using the linux version of my last version of Barbarossa (unreleased yet, but hopefully soon), together with perf.

I was surprised to see thet the branch misses are 3.05% of all branches, and the cache misses only 1,35%! In my opinion those are very good values.

I measured this during a search for depth 12 from the initial position, which for Barbarossa takes between 2 and 3 seconds. I want to try for longer searches, but beside of this, I wanted to ask those of you which have a linux box if you can post results for your engines (most are probably written in C++), to see what tipical values for engines are.

matthewlai
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Re: Tipical cache and branch misses for a chess engine

Post by matthewlai » Wed Sep 14, 2016 5:36 pm

nionita wrote:Because Barbarossa is written in Haskell, which allocates a lot of bytes all the time and then lets a garbage collecton run pretty often, I always assumed the cache misses must be very high in my engine. Also, because there is no way in Haskel to give the compiler a hint which part of a branch is expected to be more probable, I assumed that branch misses must be very high too.

But we know that our beliefs are mostly wrong and we better measure as much as possible, so this is what I did, using the linux version of my last version of Barbarossa (unreleased yet, but hopefully soon), together with perf.

I was surprised to see thet the branch misses are 3.05% of all branches, and the cache misses only 1,35%! In my opinion those are very good values.

I measured this during a search for depth 12 from the initial position, which for Barbarossa takes between 2 and 3 seconds. I want to try for longer searches, but beside of this, I wanted to ask those of you which have a linux box if you can post results for your engines (most are probably written in C++), to see what tipical values for engines are.
Most C++ programs don't give compilers branch hints, either. Modern branch predictors are very smart, and don't require hinting to work well. Most branches are very easy to predict (they always go one way).
Author of Giraffe, an engine based on deep reinforcement learning. https://bitbucket.org/waterreaction/giraffe/overview

bob
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Re: Tipical cache and branch misses for a chess engine

Post by bob » Wed Sep 14, 2016 5:51 pm

matthewlai wrote:
nionita wrote:Because Barbarossa is written in Haskell, which allocates a lot of bytes all the time and then lets a garbage collecton run pretty often, I always assumed the cache misses must be very high in my engine. Also, because there is no way in Haskel to give the compiler a hint which part of a branch is expected to be more probable, I assumed that branch misses must be very high too.

But we know that our beliefs are mostly wrong and we better measure as much as possible, so this is what I did, using the linux version of my last version of Barbarossa (unreleased yet, but hopefully soon), together with perf.

I was surprised to see thet the branch misses are 3.05% of all branches, and the cache misses only 1,35%! In my opinion those are very good values.

I measured this during a search for depth 12 from the initial position, which for Barbarossa takes between 2 and 3 seconds. I want to try for longer searches, but beside of this, I wanted to ask those of you which have a linux box if you can post results for your engines (most are probably written in C++), to see what tipical values for engines are.
Most C++ programs don't give compilers branch hints, either. Modern branch predictors are very smart, and don't require hinting to work well. Most branches are very easy to predict (they always go one way).
There are no "branch hints" in modern microprocessors, so far as I know. Intel added them in the PIV, but so far as I know, it has been removed and not re-introduced. It was a lousy idea and only matters when there is a limited branch-prediction BTB. A good optimizer, using PGO, can use the processor's static branch prediction (that which is used when there is no BTB entry to use for prediction) to make the branch fit the processor's static branch prediction algorithm better...

Your numbers are pretty much within what I would expect. You can probably improve branch prediction and cache miss rate a bit with PGO if you are not using that...

Intel's vTune is REALLY good at analyzing / collecting this kind of data.

nionita
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Re: Tipical cache and branch misses for a chess engine

Post by nionita » Wed Sep 14, 2016 6:15 pm

bob wrote:You can probably improve branch prediction and cache miss rate a bit with PGO if you are not using that...
GHC, which is best Haskell compiler, does not have (yet) PGO.
bob wrote:Intel's vTune is REALLY good at analyzing / collecting this kind of data.
I want to give it a try too, on the windows version of Barbarossa (don't know if in this case it is still free...)

nionita
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Re: Tipical cache and branch misses for a chess engine

Post by nionita » Wed Sep 14, 2016 6:20 pm

nionita wrote:
bob wrote:Intel's vTune is REALLY good at analyzing / collecting this kind of data.
I want to give it a try too, on the windows version of Barbarossa (don't know if in this case it is still free...)
Oh, I see it has support only for a few languages (and not for Haskell).

nionita
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Re: Tipical cache and branch misses for a chess engine

Post by nionita » Wed Sep 14, 2016 6:32 pm

Now I measured for depth 16, which takes between 5 and 6 seconds.

Branch misses are a bit higher (3,5%) and cache misses even less: 0,65%.

I think much better will be hard...

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