Safe FFI and Blocking IO

Tue Dec 4 18:31:31 UTC 2018

i only do unsafe vs safe ffi, most things i do i make sure that a c call
running to completion (and associated system calls) dont prevent other work
from happening. Interrupts can require resource cleanup. what sort of
examples do you have in mind?

you *could* do interruptable in lieu of safe as long as you have some way
to cleanup memory allocations i guess?
https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi-chap.html#interruptible-foreign-calls

to quote the documentation
"

interruptible behaves exactly as safe, except that when a throwTo is
directed at a thread in an interruptible foreign call, an OS-specific
mechanism will be used to attempt to cause the foreign call to return:
Unix systemsThe thread making the foreign call is sent a SIGPIPE signal
using pthread_kill(). This is usually enough to cause a blocking system
call to return with EINTR (GHC by default installs an empty signal handler
for SIGPIPE, to override the default behaviour which is to terminate the
process immediately).Windows systems[Vista and later only] The RTS calls
the Win32 function CancelSynchronousIo, which will cause a blocking I/O
operation to return with the error ERROR_OPERATION_ABORTED.

"

i dont know if the cffi overheads differ when doing an interruptable call,
and you really do have to think carefully about state cleanup... I like
avoiding needing to do state cleanup personally.

if you mean to ask about SAFE vs UNSAFE, its mostly about often the code
i'm binding has VERY predictable complexity / runtime behavior as a
function of input size, and i choose an input size threshold thats more
than ~ 1 microsecond and less than 10 microseconds.  smaller stuff gets
unsafe and larger gets safe.

Last time i measure stuff years ago, safe ffi calls had ~ 200 nanosecond
overhead on the applicable laptop, so as long as youre not doing scribbles
to unpinned memory, you should always always use the SAFE ffi for
operations that will likely take >= 1-10 microseconds ALWAYS. (theres lots
of math C code on hackage which does unsafe FFI and yet the input sizes of
interest will likely take several second to compute, a nasty combo in a
server/networked env).

tl;dr any network api / file system api, pretty safe to do safe api calls
always, afaik few to none of those are sub microsecond, heck just a memory
read from ram is 5-10 microseconds right?

On Tue, Dec 4, 2018 at 1:08 PM Andrew Martin <andrew.thaddeus at gmail.com>
wrote:

> What's your heuristic for deciding between safe and interruptible? I find
> that every time something takes long enough to warrant using the safe FFI,
> I also want to be able to kill it from a separate thread.
>
> On Tue, Dec 4, 2018 at 9:58 AM Carter Schonwald <
> carter.schonwald at gmail.com> wrote:
>
>> yup! (this is also kinda related to how the IO manager only runs on
>> -threaded built applications)
>>
>> I actually do the following pattern in some libraries i've written: bind
>> both unsafe and safe versions of a functions, and when work input is below
>> some size that i think will be less than ~ 1-10 microseconds, i do an
>> unsafe call, otherwise i do a safe call! (unsafe calls block the GC, which
>> is bad in say a server app, as you can well guess)
>>
>> the most recent and tiny example of this is a tiny sha3 implementation
>> (still need to tweak it, i think i left another 4-6x performance on the
>> table https://hackage.haskell.org/package/SecureHash-SHA3)
>>
>> On Tue, Dec 4, 2018 at 8:26 AM Andrew Martin <andrew.thaddeus at gmail.com>
>> wrote:
>>
>>> Sorry. I just found the answer to this in the manual:
>>>
>>> "When you call a foreign imported function that is annotated as safe
>>> (the default), and the program was linked using -threaded, then the call
>>> will run concurrently with other running Haskell threads. If the program
>>> was linked without -threaded, then the other Haskell threads will be
>>> blocked until the call returns."
>>>
>>> "This means that if you need to make a foreign call to a function that
>>> takes a long time or blocks indefinitely, then you should mark it safe and
>>> use -threaded. Some library functions make such calls internally; their
>>> documentation should indicate when this is the case."
>>>
>>>
>>>
>>> On Tue, Dec 4, 2018 at 8:23 AM Andrew Martin <andrew.thaddeus at gmail.com>
>>> wrote:
>>>
>>>> According to the FFI chapter [1] in the GHC manual, the safe FFI is
>>>> useful when you need to call a C function that can call back into haskell
>>>> code. I had always assumed that the scheduler could somehow interrupt safe
>>>> FFI calls, but the manual does not indicate this, and in some recent
>>>> testing I did in the posix library [2], I found that scheduling interrupts
>>>> definitely do not happen. With the non-threaded runtime, the following test
>>>> always hangs:
>>>>
>>>>     testSocketsD :: IO ()
>>>>     testSocketsD = do
>>>>       (a,b) <- demand =<< S.socketPair P.unix P.datagram
>>>> P.defaultProtocol
>>>>       _ <- forkIO $ do
>>>>         bytesSent <- demand =<< S.sendByteArray b sample 0 5 mempty
>>>>         when (bytesSent /= 5) (fail "testSocketsD: bytesSent was wrong")
>>>>       actual <- demand =<< S.receiveByteArray a 5 mempty
>>>>       actual @=? sample
>>>>
>>>>     sample :: ByteArray
>>>>     sample = E.fromList [1,2,3,4,5]
>>>>
>>>>     demand :: Either Errno a -> IO a
>>>>     demand = either (\e -> ioError (errnoToIOError "test" e Nothing
>>>> Nothing)) pure
>>>>
>>>> In the above example, sendByteArray and receiveByteArray are safe FFI
>>>> wrappers around send and recv. It is necessary to use threadWaitRead and
>>>> threadWaitWrite before these calls to predictably get the correct behavior.
>>>>
>>>> This brings to my question. In issue #34 on the github library for the
>>>> unix package [3], there is a discussion about whether to use the safe or
>>>> unsafe FFI for various POSIX system calls. On the issue there is strong
>>>> consensus that the safe FFI calls lead to better performance.
>>>>
>>>> Simon Marlow writes [4] that "Unsafe foreign imports which can block
>>>> for unpredictable amounts of time cause performance problems that only
>>>> emerge when scaling to multiple cores, because they delay the GC sync. This
>>>> is a really annoying problem if it happens to you, because it's almost
>>>> impossible to diagnose, and if it happens due to an unsafe call in a
>>>> library then it's also really hard to fix."
>>>>
>>>> And Gregory Collins adds that "If the call would ever block (and that
>>>> includes most filesystem functions) that means you want 'safe'."
>>>>
>>>> There's something I'm definitely missing. My experience is that safe
>>>> FFI calls do not help with blocking IO (again, I've been using the
>>>> non-threaded runtime, but I doubt this makes a difference), that they only
>>>> help with C functions that call back into haskell. However, a lot of other
>>>> people seem to have a difference experience.
>>>>
>>>> [1]
>>>> https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ffi-chap.html#foreign-function-interface-ffi
>>>> [2] https://github.com/andrewthad/posix
>>>> [3] https://github.com/haskell/unix/issues/34
>>>> [4] https://github.com/haskell/unix/issues/34#issuecomment-68683424
>>>>
>>>> --
>>>> -Andrew Thaddeus Martin
>>>>
>>>
>>>
>>> --
>>> -Andrew Thaddeus Martin
>>> _______________________________________________
>>> Libraries mailing list
>>> Libraries at haskell.org
>>> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>>>
>>
>
> --
> -Andrew Thaddeus Martin
>
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