Re: FreeBSD 5.3 I/O Performance / Linux 2.6.10 | Continued Discussion

From: Robert Watson (rwatson_at_freebsd.org)
Date: 01/27/05

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    Date: Thu, 27 Jan 2005 00:40:08 +0000 (GMT)
To: Mike Tancsa <mike@sentex.net>

    On Thu, 27 Jan 2005, Robert Watson wrote:

    > While it's not for the feint of heart, it might be interesting to see
    > how results compare in 6-CURRENT + debugging of various sorts (including
    > malloc) turned off, and debug.mpsafevfs turned on. One possible issue
    > with the twe/twa drivers is that they are currently MPSAFE, so may see
    > substantial contention (and hence additional latency). The move to an
    > MPSAFE VFS will help with that a lot, I should think.

    And, if you're in the mood for hacking code, and promise not to use
    snapshots, try making vfs_subr.c:vn_start_write(),
    vfs_subr.c:vn_write_suspend_wait(), vfs_subr.c:vn_finished_write(),
    vfs_subr.c:vfs_write_suspend(), and vfs_subr.c:vfs_write_resume() into
    noop's. These calls are used to avoid some deadlock scenarios associated
    with snapshot generation, but they also introduce a small but non-trivial
    amount of overhead to a number of operations. Since you're set up to do
    some testing, knowing how much of that cost is from these operations
    should be quite interesting.

    Robert N M Watson

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