Hello Simon, Wednesday, May 3, 2006, 3:07:19 PM, you wrote:
I propose to add INLINE pragmas to withMVar and friends.
and to any higher-order function? :) imho, key of this problem is that GHC don't have a way to optimize just the required function with all it's enclosed calls. in old GHC papers there is an "inline" function that does this (afai understood) but it is no more supported problem is not that withMVar should be inlined in any case of its use. problem is what i need to inline it's use in one concrete place of my concrete library and there is no way to force GHC do this beside of copying it's implementation! i think that GHC should just give more priority to inlining higher-order and dictionaries-dependent functions (afaiu, currently they are weighted by the same rules as ordinary functions) and the second part of problem is what it can't be decided at the place of "withMVar" definition whether it should be inlined or not - this depends in the first place on the call site. if this is the code that performed just one time, we should decide on the space basis. if this is the innermost loop, we should inline even largest functions (especially if they are higher-order/polymorphic ones). but the compiler don't know how much times some code fragment will be performed (obviously it's place for future integration between optimizer and profiler...) so, i think, it should generally optimize program on code space and only for procedures marked as OPTIMIZE it should enable all the inlining machinery, trying to make code generated for this procedure (including all the inner calls) as fast as possible. this should allow programmer to just mark speed-critical procedures and will make library writers absolutely free from dancing with INLINE pragmas. while currently to optimize some procedure in my application, i mark it INLINE, then find and mark all procedures it call, and further recursively. i prefer that GHC will do it instead of me :)
Having an interface for simple locks sounds like a good idea to me. Would you like to send a patch?
i've attached this module together with two examples on that i've tested it. first example (a.hs) really don't work :) i think it's shortcoming of current Haskell standard (at least, Hugs also can't compile it). moreover, it seems that we should change MArray declaration to the following: class (Monad m, MutableBounds a m, Ix i) => MArray a e m | a->e, a->i where newArray :: (i,i) -> e -> m a newArray_ :: (i,i) -> m a unsafeRead :: a -> Int -> m e unsafeWrite :: a -> Int -> e -> m () (i.e. 'a' here may be for example "IOArray Int Double" while it's just "IOArray" in current definition) drawback is what this needs FD (while old definition use only MPTC) and what this new definition will be not compatible with some old code. on the other side, as you can see here, using of type constructor in class definition tend to create problem with partially-applied type functions, i've encountered this problem several times while redesigning array library, but before this moment each time i found some way around problem. but the problem still exists...
This won't affect Handle I/O unfortunately, because we need block to protect against asynchronous exceptions.
i think that it is not accidental that hPutChar/hGetChar is exactly 3 and 2 times slower than their v* analogs. my routines use only one locked MVar and i think that Handle routines used 3 and 2 mvars, respectively. i thought that they call withMVar directly, but it is not the case. but may be great blocks that they are perform inside "block" operations are still not inlined and therefore suffers from the same problem as withMVar? i guess that adding INLINE pragma to block definition may speed up their execution (in 52/38 ~= 1.4 times), but possibly at the cost of code size. anyway, Handle I/O is already too complicated thing to worry about it's further optimizations :(
I'm still not certain you won't need that in the stream library, too: check any stateful code (eg. buffering) and imagine what happens if an exception is raised at an arbitrary point.
thank, it's sort of critics what i very need. the total checking seems to rather complex problem so i will decline it until working on next release. the good news is what my buffering scheme updates only one unboxed pointer (current read/write position inside buffer) on all operations that can be performed using only buffer contents, so it seems that i don't need to use "block" to protect such operations until routine realizes that there is need to switch to other buffer. btw, i've changed default buffer size to 4kb in next version i will work on this problem. i think that it should be described as some contract between library authors (including authors of 3rd-party streams and transformers) and users, in this form: if stream operation abandoned because of asynchronous interrupt, internal bug or error in input data: - it should leave stream in coherent state (f.e., it should not leave vIsEOF=True and vTell=0 for non-empty-file) - it should not have "side-effects", f.e. changing position if whole operation (such as vShow) don't have such effect - operation can be performed only partially (i.e. vSeek operation can leave pointer in unpredictable place, vPutStr can output only part of string, vGetBuf may fill only part of buffer) - if possible, operation implementation should give to async. exceptions chances to interrupt execution with rather small granularity (i.e. don't write 100 mb in one C call, but split it to 4 kb blocks) what you will say? -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com