Re: Mixed boxed/unboxed arrays?
On Tue, Aug 02, 2022 at 05:32:58PM +0200, J. Reinders wrote: > > Could you use `StablePtr` for the keys? > > That might be an option, but I have no idea how performant stable > pointers are and manual management is obviously not ideal. If your hash table keys qualify for being stored in a "compact region", you may not need per-key stable pointers, just (carefully) coercing the keys to pointers suffices to produce primitive "handles" that are stable for the lifetime of the "compact region". The inverse (unsafe) coercion recovers the key. This also has the advantage that a key count does not incur a high ongoing GC cost. The keys are of course copied into the compact region. With this you could store "pointer + count" in a primitive cell. The hash table then holds a reference to the compact region and compacts keys on insert. https://hackage.haskell.org/package/compact-0.2.0.0/docs/Data-Compact.html -- Viktor. ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Mixed boxed/unboxed arrays?
Indeed I misunderstood. As you already suspected this wouldn't work for
Int# (or other unboxed types) sadly as the GC would assume these to be
pointers which no doubt would lead to segfaults or worse.
Rereading your initial mail I can say the runtime currently doesn't
support such a heap object.
If I understand you correctly what you would like is basically a
something like:
Con n P I# P I# P I# ...
\/ \/\/
Pair1 Pair2 Pair3 ...
Where n gives the number of pairs.
I can see how it might be feasible to add a heap object like this to GHC
but I'm unsure if it would be worth the complexity as it's layout
diverges quite a bit from what GHC usually expects.
The other option would be to expose to users a way to have an object
that consist of a given number of words and a bitmap which indicates to
the GHC which fields are pointers. This is more or less
the representation that's already used to deal with stack frames iirc so
that might not be as far fetched as it seems at first.
It might even be possible to implement some sort of prototype for this
using hand written Cmm.
But there are not any plans to implement anything like this as far as I
know.
Am 02/08/2022 um 20:51 schrieb Jaro Reinders:
It seems you have misunderstood me. I want to store *unboxed* Int#s
inside the array, not just some unlifted types. Surely in the case of
unboxed integers the unsafeCoerce# function can make the garbage
collector crash as they might be interpreted as arbitrary pointers.
Cheers,
Jaro
On 02/08/2022 20:24, Andreas Klebinger wrote:
I think it's possible to do this *today* using unsafeCoerce#.
I was able to come up with this basic example below. In practice one
would at the very least want to abstract away the gnarly stuff inside a
library. But since it sounds like you want to be the one to write a
library that shouldn't be a problem.
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE UnliftedDatatypes #-}
moduleMainwhere
importGHC.Exts
importGHC.IO
importUnsafe.Coerce
importData.Kind
dataSA= SA (SmallMutableArray# RealWorldAny)
mkArray:: Int-> a-> IO(SA)
mkArray (I# n) initial = IO $ \s ->
caseunsafeCoerce# (newSmallArray# n initial s) of
(# s', arr #) -> (# s', SA arr #)
readLifted:: SA-> Int-> IOa
readLifted (SA arr) (I# i) = IO (\s ->
unsafeCoerce# (readSmallArray# arr i s)
)
dataUWrap(a:: UnliftedType) = UWrap a
-- UWrap is just here because we can't return unlifted types in IO
-- If you don't need your result in IO you can eliminate this
indirection.
readUnlifted:: foralla. SA-> Int-> IO(UWrapa)
readUnlifted (SA arr) (I# i) = IO (\s ->
caseunsafeCoerce# (readSmallArray# arr i s) of
(# s', a :: a#) -> (# s', UWrap a #)
)
writeLifted:: a-> Int-> SA-> IO()
writeLifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# (unsafeCoerce# arr) i x s of
s -> (# s, ()#)
writeUnlifted:: (a:: UnliftedType) -> Int-> SA-> IO()
writeUnlifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# arr i (unsafeCoerce# x) s of
s -> (# s, ()#)
typeUB:: UnliftedType
dataUB= UT | UF
showU:: UWrapUB-> String
showU (UWrap UT) = "UT"
showU (UWrap UF) = "UF"
main:: IO()
main = do
arr <- mkArray 4()
writeLifted True 0arr
writeLifted False 1arr
writeUnlifted UT 2arr
writeUnlifted UT 3arr
(readLifted arr 0:: IOBool) >>= print
(readLifted arr 1:: IOBool) >>= print
(readUnlifted arr 2:: IO(UWrapUB)) >>= (putStrLn . showU)
(readUnlifted arr 3:: IO(UWrapUB)) >>= (putStrLn . showU)
return ()
Cheers
Andreas
Am 02/08/2022 um 17:32 schrieb J. Reinders:
Could you use `StablePtr` for the keys?
That might be an option, but I have no idea how performant stable pointers are
and manual management is obviously not ideal.
How does the cost of computing object hashes and comparing colliding
objects compare with the potential cache miss cost of using boxed
integers or a separate array? Would such an "optimisation" be worth
the effort?
Literature on hash tables suggests that cache misses were a very important
factor in running time (in
2001):https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.4189
I don’t know whether it has become less or more important now, but I have been
told there haven’t been that many advances in memory latency.
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Re: Mixed boxed/unboxed arrays?
It seems you have misunderstood me. I want to store *unboxed* Int#s
inside the array, not just some unlifted types. Surely in the case of
unboxed integers the unsafeCoerce# function can make the garbage
collector crash as they might be interpreted as arbitrary pointers.
Cheers,
Jaro
On 02/08/2022 20:24, Andreas Klebinger wrote:
I think it's possible to do this *today* using unsafeCoerce#.
I was able to come up with this basic example below. In practice one
would at the very least want to abstract away the gnarly stuff inside a
library. But since it sounds like you want to be the one to write a
library that shouldn't be a problem.
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE UnliftedDatatypes #-}
moduleMainwhere
importGHC.Exts
importGHC.IO
importUnsafe.Coerce
importData.Kind
dataSA= SA (SmallMutableArray# RealWorldAny)
mkArray:: Int-> a-> IO(SA)
mkArray (I# n) initial = IO $ \s ->
caseunsafeCoerce# (newSmallArray# n initial s) of
(# s', arr #) -> (# s', SA arr #)
readLifted:: SA-> Int-> IOa
readLifted (SA arr) (I# i) = IO (\s ->
unsafeCoerce# (readSmallArray# arr i s)
)
dataUWrap(a:: UnliftedType) = UWrap a
-- UWrap is just here because we can't return unlifted types in IO
-- If you don't need your result in IO you can eliminate this indirection.
readUnlifted:: foralla. SA-> Int-> IO(UWrapa)
readUnlifted (SA arr) (I# i) = IO (\s ->
caseunsafeCoerce# (readSmallArray# arr i s) of
(# s', a :: a#) -> (# s', UWrap a #)
)
writeLifted:: a-> Int-> SA-> IO()
writeLifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# (unsafeCoerce# arr) i x s of
s -> (# s, ()#)
writeUnlifted:: (a:: UnliftedType) -> Int-> SA-> IO()
writeUnlifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# arr i (unsafeCoerce# x) s of
s -> (# s, ()#)
typeUB:: UnliftedType
dataUB= UT | UF
showU:: UWrapUB-> String
showU (UWrap UT) = "UT"
showU (UWrap UF) = "UF"
main:: IO()
main = do
arr <- mkArray 4()
writeLifted True 0arr
writeLifted False 1arr
writeUnlifted UT 2arr
writeUnlifted UT 3arr
(readLifted arr 0:: IOBool) >>= print
(readLifted arr 1:: IOBool) >>= print
(readUnlifted arr 2:: IO(UWrapUB)) >>= (putStrLn . showU)
(readUnlifted arr 3:: IO(UWrapUB)) >>= (putStrLn . showU)
return ()
Cheers
Andreas
Am 02/08/2022 um 17:32 schrieb J. Reinders:
Could you use `StablePtr` for the keys?
That might be an option, but I have no idea how performant stable pointers are
and manual management is obviously not ideal.
How does the cost of computing object hashes and comparing colliding
objects compare with the potential cache miss cost of using boxed
integers or a separate array? Would such an "optimisation" be worth
the effort?
Literature on hash tables suggests that cache misses were a very important
factor in running time (in
2001):https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.4189
I don’t know whether it has become less or more important now, but I have been
told there haven’t been that many advances in memory latency.
___
ghc-devs mailing list
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Re: Mixed boxed/unboxed arrays?
I think it's possible to do this *today* using unsafeCoerce#.
I was able to come up with this basic example below. In practice one
would at the very least want to abstract away the gnarly stuff inside a
library. But since it sounds like you want to be the one to write a
library that shouldn't be a problem.
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE UnliftedDatatypes #-}
moduleMainwhere
importGHC.Exts
importGHC.IO
importUnsafe.Coerce
importData.Kind
dataSA= SA (SmallMutableArray# RealWorldAny)
mkArray:: Int-> a-> IO(SA)
mkArray (I# n) initial = IO $ \s ->
caseunsafeCoerce# (newSmallArray# n initial s) of
(# s', arr #) -> (# s', SA arr #)
readLifted:: SA-> Int-> IOa
readLifted (SA arr) (I# i) = IO (\s ->
unsafeCoerce# (readSmallArray# arr i s)
)
dataUWrap(a:: UnliftedType) = UWrap a
-- UWrap is just here because we can't return unlifted types in IO
-- If you don't need your result in IO you can eliminate this indirection.
readUnlifted:: foralla. SA-> Int-> IO(UWrapa)
readUnlifted (SA arr) (I# i) = IO (\s ->
caseunsafeCoerce# (readSmallArray# arr i s) of
(# s', a :: a#) -> (# s', UWrap a #)
)
writeLifted:: a-> Int-> SA-> IO()
writeLifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# (unsafeCoerce# arr) i x s of
s -> (# s, ()#)
writeUnlifted:: (a:: UnliftedType) -> Int-> SA-> IO()
writeUnlifted x (I# i) (SA arr) = IO $ \s ->
casewriteSmallArray# arr i (unsafeCoerce# x) s of
s -> (# s, ()#)
typeUB:: UnliftedType
dataUB= UT | UF
showU:: UWrapUB-> String
showU (UWrap UT) = "UT"
showU (UWrap UF) = "UF"
main:: IO()
main = do
arr <- mkArray 4()
writeLifted True 0arr
writeLifted False 1arr
writeUnlifted UT 2arr
writeUnlifted UT 3arr
(readLifted arr 0:: IOBool) >>= print
(readLifted arr 1:: IOBool) >>= print
(readUnlifted arr 2:: IO(UWrapUB)) >>= (putStrLn . showU)
(readUnlifted arr 3:: IO(UWrapUB)) >>= (putStrLn . showU)
return ()
Cheers
Andreas
Am 02/08/2022 um 17:32 schrieb J. Reinders:
Could you use `StablePtr` for the keys?
That might be an option, but I have no idea how performant stable pointers are
and manual management is obviously not ideal.
How does the cost of computing object hashes and comparing colliding
objects compare with the potential cache miss cost of using boxed
integers or a separate array? Would such an "optimisation" be worth
the effort?
Literature on hash tables suggests that cache misses were a very important
factor in running time (in
2001):https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.4189
I don’t know whether it has become less or more important now, but I have been
told there haven’t been that many advances in memory latency.
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ghc-devs@haskell.org
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Re: [ANNOUNCE] GHC 9.4.1-rc1 is now available
George, Kazu, I also can't reproduce on the mac which I can access over SSH. I downloaded the bindist for 9.2.4 and 9.4.1-rc1 and could install them both and run the binaries. Matt On Mon, Jul 25, 2022 at 5:23 AM Kazu Yamamoto (山本和彦) via ghc-devs wrote: > > Hi George, > > > I've duplicated the issue on both of my machines. It would be good to know > > if anybody else is seeing it. Kazu, I know you have seen this in the past. > > Do you get the same error installing rc1? > > When I run sudo make install I get a popup that says: > > I had no problem on 9.4.1-rc1. > "xattr -rc ." and "make install" worked perfectly. > > macOS Monterey v12.4 > Xcode 13.4.1 > > --Kazu > > > ___ > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Mixed boxed/unboxed arrays?
> Could you use `StablePtr` for the keys? That might be an option, but I have no idea how performant stable pointers are and manual management is obviously not ideal. > How does the cost of computing object hashes and comparing colliding > objects compare with the potential cache miss cost of using boxed > integers or a separate array? Would such an "optimisation" be worth > the effort? Literature on hash tables suggests that cache misses were a very important factor in running time (in 2001): https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.25.4189 I don’t know whether it has become less or more important now, but I have been told there haven’t been that many advances in memory latency. ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Re: Mixed boxed/unboxed arrays?
On Tue, Aug 02, 2022 at 03:31:57PM +0200, J. Reinders wrote: > I’ve been investigating fast hash table implementations. In particular > hash tables used for counting unique items. For this use case, I > believe the most performant hash tables are, in C terms, arrays of > structures with a (boxed) pointer to the key, which is the item that > we are counting, and an (unboxed) integer which holds the actual > count. > > I already know of the ‘vector-hashtables’ package which uses two > separate arrays, for example one boxed to hold the keys and one > unboxed to hold the counts. However, I believe it can be quite > important to store all the elements in the same array as that can > reduce the number of cache misses. Because with random access to two > arrays there is a higher chance that there will be two cache misses > even if it immediately finds the right key in the hash table. Could you use `StablePtr` for the keys? https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.16.1.0/GHC-Stable.html The corresponding `Ptr` can be stored in an unboxed Storable array along with the count. This comes at the cost of later having to explicitly free each StablePtr. https://downloads.haskell.org/~ghc/latest/docs/html/libraries/base-4.16.1.0/GHC-Stable.html#v:freeStablePtr How does the cost of computing object hashes and comparing colliding objects compare with the potential cache miss cost of using boxed integers or a separate array? Would such an "optimisation" be worth the effort? -- Viktor. ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
Mixed boxed/unboxed arrays?
Hi GHC devs, I’ve been investigating fast hash table implementations. In particular hash tables used for counting unique items. For this use case, I believe the most performant hash tables are, in C terms, arrays of structures with a (boxed) pointer to the key, which is the item that we are counting, and an (unboxed) integer which holds the actual count. I already know of the ‘vector-hashtables’ package which uses two separate arrays, for example one boxed to hold the keys and one unboxed to hold the counts. However, I believe it can be quite important to store all the elements in the same array as that can reduce the number of cache misses. Because with random access to two arrays there is a higher chance that there will be two cache misses even if it immediately finds the right key in the hash table. So, I have also been looking at the low level arrays from the ‘primitive’ package and even in GHC.Exts, but I don’t believe it is currently possible to create a single array that contains both boxed and unboxed elements. Have I overlooked something? Or else, would it be possible to support this use case in a future version of GHC? Cheers, Jaro ___ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
