Re: 1352 NUL bytes at the end of a page? (was Re: Assertion `s && s->tree' failed: The saga continues.)

From: Steven Cole (elenstev_at_mesatop.com)
Date: 05/16/04

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    To: Andrea Arcangeli <andrea@suse.de>
Date:	Sun, 16 May 2004 09:28:21 -0600

    On Saturday 15 May 2004 11:22 pm, Andrea Arcangeli wrote:
    > On Sat, May 15, 2004 at 09:52:50PM -0700, Linus Torvalds wrote:
    > >
    > >
    > > On Sat, 15 May 2004, Steven Cole wrote:
    > > >
    > > > OK, will do. I ran the bk exerciser script for over an hour with 2.6.6-current
    > > > and no CONFIG_PREEMPT and no errors. The script only reported one
    > > > iteration finished, while I got it to do 36 iterations over several hours earlier
    > > > today (with a 2.6.3-4mdk vendor kernel)
    > >
    > > Hmm.. Th ecurrent BK tree contains much of the anonvma stuff, so this
    > > might actually be a serious VM performance regression. That could
    > > effectively be hiding whatever problem you saw.
    > >
    > > Andrea: have you tested under low memory and high fs load? Steven has 384M
    > > or RAM, which _will_ cause a lot of VM activity when doing a full kernel
    > > BK clone + undo + pull, which is what his test script ends up doing...
    >
    > An easy way to verify for Steven is to give a quick spin to 2.6.5-aa5
    > and see if it's slow too, that will rule out the anon-vma changes
    > (for completeness: there's a minor race in 2.6.5-aa5 fixed in my current
    > internal tree, I posted the fix to l-k separately, but you can ignore
    > the fix for a simple test, it takes weeks to trigger anyways and you
    > need threads to trigger it and I've never seen threaded version control
    > systems so I doubt BK is threaded).

    I'm getting the linux-2.6.5.tar.bz2 file (already got 2.6.5-aa2) via ppp,
    while running the bk test script on 2.6.6-current and no PREEMPT.
    That takes a while on 56k dialup. I'll leave all that running while
    I go hiking.

    >
    > In general a "slowdown" cannot be related to anon-vma (unless it's a
    > minor merging error), that's a black and white thing, it doesn't touch
    > the vm heuristics and it will only speed the fast paths up plus it will
    > save some tons of ram in the big systems. Pratically no change should be
    > measurable on a small system (unless it uses an heavy amount of cows, in
    > which case it will improve things, it should never hurt). As for being
    > tested, it is very well tested on the small desktops too. Probably the
    > only thing to double check is that there was no minor merging error that
    > could have caused this.

    Andrea, I did see a significant slowdown with Andy's test script (with DMA on)
    on my timed test of 2.6.6-current vs 2.6.3.

    >
    > > It would be good to test going back to the kernel that saw the "immediate
    > > problem", and try that version without CONFIG_PREEMPT.
    >
    > Agreed.
    >
    > Thanks.
    >
    >

    Yep, later this evening, I hope.

    Steven
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