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 <[email protected]
<mailto:[email protected]>> 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 <[email protected] <mailto:[email protected]>>
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