[Haskell-cafe] File I/O benchmark help (conduit, io-streams and Handle)

[Simon Marlow]

1GB/s for copying a file is reasonable - it's around half the memory 
bandwidth, so copying the data twice would give that result (assuming no 
actual I/O is taking place, which is what you want because actual I/O 
will swamp any differences at the software level).

The Handle overhead should be negligible if you're only using 
hGetBufSome and hPutBuf, because those functions basically just call 
read() and write() when the amount of data is larger than the buffer size.

There's clearly something suspicious going on here, unfortunately I 
don't have time right now to investigate, but I'll keep an eye on the 
thread.

Cheers,
	Simon

On 08/03/13 08:36, Gregory Collins wrote:
> +Simon Marlow
> A couple of comments:
>
>   * maybe we shouldn't back the file by a Handle. io-streams does this
>     by default out of the box; I had a posix file interface for unix
>     (guarded by CPP) for a while but decided to ditch it for simplicity.
>     If your results are correct, given how slow going by Handle seems to
>     be I may revisit this, I figured it would be "good enough".
>   * io-streams turns Handle buffering off in withFileAsOutput. So the
>     difference shouldn't be as a result of buffering. Simon: is this an
>     expected result? I presume you did some Handle debugging?
>   * the IO manager should not have any bearing here because file code
>     doesn't actually ever use it (epoll() doesn't work for files)
>   * does the difference persist when the file size gets bigger?
>   * your file descriptor code doesn't handle EINTR properly, although
>     you said you checked that the file copy is being done?
>   * Copying a 1MB file in 1ms gives a throughput of ~1GB/s. The other
>     methods have a more believable ~70MB/s throughput.
>
> G
>
>
> On Fri, Mar 8, 2013 at 7:30 AM, Michael Snoyman <michael at snoyman.com
> <mailto:michael at snoyman.com>> wrote:
>
>     Hi all,
>
>     I'm turning to the community for some help understanding some
>     benchmark results[1]. I was curious to see how the new io-streams
>     would work with conduit, as it looks like a far saner low-level
>     approach than Handles. In fact, the API is so simple that the entire
>     wrapper is just a few lines of code[2].
>
>     I then added in some basic file copy benchmarks, comparing
>     conduit+Handle (with ResourceT or bracket), conduit+io-streams,
>     straight io-streams, and lazy I/O. All approaches fell into the same
>     ballpark, with conduit+bracket and conduit+io-streams taking a
>     slight lead. (I haven't analyzed that enough to know if it means
>     anything, however.)
>
>     Then I decided to pull up the NoHandle code I wrote a while ago for
>     conduit. This code was written initially for Windows only, to work
>     around the fact that System.IO.openFile does some file locking. To
>     avoid using Handles, I wrote a simple FFI wrapper exposing open,
>     read, and close system calls, ported it to POSIX, and hid it behind
>     a Cabal flag. Out of curiosity, I decided to expose it and include
>     it in the benchmark.
>
>     The results are extreme. I've confirmed multiple times that the copy
>     algorithm is in fact copying the file, so I don't think the test
>     itself is cheating somehow. But I don't know how to explain the
>     massive gap. I've run this on two different systems. The results you
>     see linked are from my local machine. On an EC2 instance, the gap
>     was a bit smaller, but the NoHandle code was still 75% faster than
>     the others.
>
>     My initial guess is that I'm not properly tying into the IO manager,
>     but I wanted to see if the community had any thoughts. The relevant
>     pieces of code are [3][4][5].
>
>     Michael
>
>     [1] http://static.snoyman.com/streams.html
>     [2]
>     https://github.com/snoyberg/conduit/blob/streams/io-streams-conduit/Data/Conduit/Streams.hs
>     [3]
>     https://github.com/snoyberg/conduit/blob/streams/conduit/System/PosixFile.hsc
>     [4]
>     https://github.com/snoyberg/conduit/blob/streams/conduit/Data/Conduit/Binary.hs#L54
>     [5]
>     https://github.com/snoyberg/conduit/blob/streams/conduit/Data/Conduit/Binary.hs#L167
>
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>
> --
> Gregory Collins <greg at gregorycollins.net <mailto:greg at gregorycollins.net>>