Re: glibc aio performance

["Don Capps" <capps at rsn dot hp dot com>]

Amos,

    I believe that this may be a case of pilot error :-)

    In the case where you ran
        iozone -i0 -i1 -k128

    You asked for Iozone to use POSIX async I/O and
    to use 128 async writes/reads of 4k and wrote/read
    a file that was 512 Kbytes in size.
    Thus.. there are 128 async read threads doing exactly
    one 4k operation.

    Let's look under the hood.
    If one spawns 128 async reads/writes then this is the
    equivalent of calling fork() 128 times and having
    the new process do a single 4k operation and then
    terminate. The overhead of creating the threads,
    and terminating the threads is eating your lunch.

    Here are some suggestions that you might find useful.

    Try using a file size that is much larger.
        Example: -s 300M
    Try using a transfer size that is larger.
        Example: -r 64
    Try using fewer async ops.
        Example: -k 32

    In the examples above the threads will do a more
    significant amount of work and they will be
    re-used many times before they terminate.

    There would be 32 threads and each thread will
    do 64 kbyte transfers. The total number of
    transfers will be 4800. Each thread will now
    do 150 ops before it terminates.

    Here is output from my Redhat  7.2 box.

    ./iozone -r 64 -s 300M -i 0 -i 1
      KB      reclen   write    rewrite    read    reread
     307200      64   37314   29174    28042    28057


    ./iozone -k 32 -r 64 -s 200M -i 0 -i 1
     KB       reclen   write    rewrite    read     reread
    307200       64   25793   30625    27376    27335

    Since the filesystem is on a single disk drive there
    is no advantage to the async operations. But, the
    result is well within reason.

    I don't believe that there is anything wrong with
    glibc or with Iozone but more of a case of getting
    what you asked for and finding out that the question
    was probably not a good one :-)

    The moral of the story is: If you are going to spawn
    a thread then it would be wise to have it do some
    significant work before it terminates.

    Hope this helps,
    Don Capps


----- Original Message -----
From: "Amos P Waterland" <waterland@us.ibm.com>
To: <libc-alpha@sources.redhat.com>
Cc: <capps@iozone.org>; <wnorcott@us.oracle.com>; <tom_gall@vnet.ibm.com>
Sent: Thursday, May 30, 2002 1:01 PM
Subject: glibc aio performance


> Hello, I have been looking at the glibc asynchronous I/O implementation,
> and have run into a bit of an issue.
>
> When I run IOzone (an open source filesystem benchmark tool) with AIO
> enabled, it reports write KB/s throughput on the order of 45 times slower
> than that reported without AIO.
>
> % iozone -i0 -i1 #run just the write and read tests
> [snip]
> KB  reclen   write rewrite    read    reread [snip]
> 512       4  101769  178334   331595   345702
> % iozone -i0 -i1 -k128  #do same, but use no-bcopy aio
> [snip]
> KB  reclen   write rewrite    read    reread [snip]
> 512       4    2232   47210   121874   103372
>
> I have looked at the source code for the glibc implementation, and it is
> not obvious to me why keeping a thread pool, each of whose contituents
> perform a pread(2) or pwrite(2), should be so much slower than synchronous
> I/O.  I looked at the source code for IOzone, and found that it uses
> libasync.c for AIO, but could find no obvious performance problems in its
> code.
>
> So my question is: Might there be a problem with the way IOzone is using
> glibc's implementation of AIO, or is glibc's implementation known to have
> performance problems?
>
> Amos Waterland
>
>