Re: [9fans] 8 cores - Plan9

This is a discussion on Re: [9fans] 8 cores - Plan9 ; > > > I could imagine that databases use mmap() havily > > > > it's a little mystery for me why they would do that since it's slower (or ought to be), > > slower compared to what? I'd ...

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Thread: Re: [9fans] 8 cores

  1. Re: [9fans] 8 cores

    > > > I could imagine that databases use mmap() havily
    > >
    > > it's a little mystery for me why they would do that since it's slower (or ought to be),

    >
    > slower compared to what? I'd expect the biggest slowdown for
    > read()/write() be not the price of a syscall, but what you
    > pay for copying data in/out of the kernel. With mmap() there's
    > no copying.


    as you've pointed out, performance-wise it's not copying vs. nothing
    it's copying vs page faults and trips through the vm code.
    i would think playing vm games (as linus likes to say) would make
    scheduling on mp harder

    - erik


  2. Re: [9fans] 8 cores

    On Thu, Jul 17, 2008 at 08:41:21AM -0400, erik quanstrom wrote:
    > as you've pointed out, performance-wise it's not copying vs. nothing
    > it's copying vs page faults and trips through the vm code.
    > i would think playing vm games (as linus likes to say) would make
    > scheduling on mp harder


    Munmap is what is really expensive. The aio stuff exists
    under Unix for Oracle and DB2, which is why it only works
    for ordinary files in direct I/O mode under Linux.

    -WJ


  3. Re: [9fans] 8 cores

    On Thu, Jul 17, 2008 at 5:41 AM, erik quanstrom wrote:

    > as you've pointed out, performance-wise it's not copying vs. nothing
    > it's copying vs page faults and trips through the vm code.
    > i would think playing vm games (as linus likes to say) would make
    > scheduling on mp harder
    >


    But you make trips through the vm code on read/write in any event,
    don't you? There was a pretty good paper comparing these paths once
    and in the end it boiled down to "your cost will vary depending on how
    you wrote the kernel" :-)

    This problem has been worked for a long time by Sun among others.
    They've had several decades to think on it.

    ron


  4. Re: [9fans] 8 cores

    > But you make trips through the vm code on read/write in any event,
    > don't you? There was a pretty good paper comparing these paths once
    > and in the end it boiled down to "your cost will vary depending on how
    > you wrote the kernel" :-)


    on plan 9? where?

    - erik



  5. Re: [9fans] 8 cores

    On Thu, Jul 17, 2008 at 6:46 AM, erik quanstrom wrote:
    >> But you make trips through the vm code on read/write in any event,
    >> don't you? There was a pretty good paper comparing these paths once
    >> and in the end it boiled down to "your cost will vary depending on how
    >> you wrote the kernel" :-)

    >
    > on plan 9? where?


    I can't remember ...

    ron


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