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    SubjectRe: [Scst-devel] [Iscsitarget-devel] ISCSI-SCST performance (with also IET and STGT data)
    James Bottomley, on 04/02/2009 12:23 AM wrote:
    > On Wed, 2009-04-01 at 08:20 -0400, Ross Walker wrote:
    >> On Apr 1, 2009, at 2:29 AM, Bart Van Assche <>
    >> wrote:
    >>> On Tue, Mar 31, 2009 at 8:43 PM, Ross S. W. Walker
    >>> <> wrote:
    >>>> IET just needs to fix how it does it workload with CFQ which
    >>>> somehow SCST has overcome. Of course SCST tweaks the Linux kernel to
    >>>> gain some extra speed.
    >>> I'm not familiar with the implementation details of CFQ, but I know
    >>> that one of the changes between SCST 1.0.0 and SCST 1.0.1 is that the
    >>> default number of kernel threads of the scst_vdisk kernel module has
    >>> been increased to 5. Could this explain the performance difference
    >>> between SCST and IET for FILEIO and BLOCKIO ?
    >> Thank for the update. IET has used 8 threads per target for ages now,
    >> I don't think it is that.
    >> It may be how the I/O threads are forked in SCST that causes them to
    >> be in the same I/O context with each other.
    >> I'm pretty sure implementing a version of the patch that was used for
    >> the dump command (found on the LKML) will fix this.
    >> But thanks goes to Vlad for pointing this dificiency out so we can fix
    >> it to help make IET even better.
    > SCST explicitly fiddles with the io context to get this to happen. It
    > has a hack to block to export alloc_io_context:

    Correct, although I wouldn't call it "fiddle", rather "grouping" ;)

    But that's not the only reason for good performance. Particularly, it
    can't explain Bart's tmpfs results from the previous message, where the
    majority of I/O done to/from RAM without any I/O scheduler involved. (Or
    does I/O scheduler also involved with tmpfs?) Bart has 4GB RAM, if I
    remember correctly, i.e. the test data set was 25% of RAM.


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