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    SubjectRe: splice/vmsplice performance test results
    On Thu, Nov 16 2006, Jim Schutt wrote:
    > Hi,
    > I've done some testing to see how splice/vmsplice perform
    > vs. other alternatives on transferring a large file across
    > a fast network. One option I tested was to use vmsplice
    > to get a 1-copy receive, but it didn't perform as well
    > as I had hoped. I was wondering if my results were at odds
    > with what other people have observed.
    > I've two systems, each with:
    > Tyan S2895 motherboard
    > 2 ea. 2.6 GHz Opteron
    > 1 GiB memory
    > Myricom Myri-10G 10 Gb/s NIC (PCIe x8)
    > 2.6.19-rc5-g134a11f0 on FC4
    > In addition, one system has a 3ware 9590-8ML (PCIe) and a 3ware
    > 9550SX-8LP (PCI-X), with 16 Seagate Barracuda 7200.10 SATA drives
    > (250 GB ea., NCQ enabled). Write caching is enabled on the 3ware
    > cards.
    > The Myricom cards are connected back-to-back using 9000 byte MTU.
    > I baseline the network performance with 'iperf -w 1M -l 64K'
    > and get 6.9 Gb/s.
    > After a fair amount of testing, I settled on a 4-way software
    > RAID0 on top of 4-way hardware RAID0 units as giving the best
    > streaming performance. The file system is XFS, with the stripe
    > unit set to the hardware RAID chunk size, and the stripe width
    > 16 times that.
    > Disk tuning parameters in /sys/block/sd*/queue are default
    > values, except queue/nr_requests = 5 gives me best performance.
    > (It seems like the 3ware cards slow down a little if I feed them
    > too much data on the streaming write test I'm using.)
    > I baseline file write performance with
    > sync; time { dd if=/dev/zero of=./zero bs=32k count=512k; sync; }
    > and get 465-520 MB/s (highly variable).
    > I test baseline file read performance with
    > time dd if=./zero of=/dev/null bs=32k count=512k
    > and get 950 MB/s (fairly repeatable).
    > My test program can do one of the following:
    > send data:
    > A) read() from file into buffer, write() buffer into socket
    > B) mmap() section of file, write() that into socket, munmap()
    > C) splice() from file to pipe, splice() from pipe to socket
    > receive data:
    > 1) read() from socket into buffer, write() buffer into file
    > 2) ftruncate() to extend file, mmap() new extent, read()
    > from socket into new extent, munmap()
    > 3) read() from socket into buffer, vmsplice() buffer to
    > pipe, splice() pipe to file (using the double-buffer trick)
    > Here's the results, using:
    > - 64 KiB buffer, mmap extent, or splice
    > - 1 MiB TCP window
    > - 16 GiB data sent across network
    > A) from /dev/zero -> 1) to /dev/null : 857 MB/s (6.86 Gb/s)
    > A) from file -> 1) to /dev/null : 472 MB/s (3.77 Gb/s)
    > B) from file -> 1) to /dev/null : 366 MB/s (2.93 Gb/s)
    > C) from file -> 1) to /dev/null : 854 MB/s (6.83 Gb/s)
    > A) from /dev/zero -> 1) to file : 375 MB/s (3.00 Gb/s)
    > A) from /dev/zero -> 2) to file : 150 MB/s (1.20 Gb/s)
    > A) from /dev/zero -> 3) to file : 286 MB/s (2.29 Gb/s)
    > I had (naively) hoped the read/vmsplice/splice combination would
    > run at the same speed I can write a file, i.e. at about 450 MB/s
    > on my setup. Do any of my numbers seem bogus, so I should look
    > harder at my test program?

    Could be read-ahead playing in here, I'd have to take a closer look at
    the generated io patterns to say more about that. Any chance you can
    capture iostat or blktrace info for such a run to compare that goes to
    the disk? Can you pass along the test program?

    > Or is read+write really the fastest way to get data off a
    > socket and into a file?

    splice() should be just as fast of course, and more efficient. Not a lot
    of real-life performance tuning has gone into it yet, so I would not be
    surprised if we need to smoothen a few edges.

    Jens Axboe

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