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    SubjectRe: 2.6.1-rc1-tiny2
    On Wed, Jan 07 2004, Matt Mackall wrote:
    > On Wed, Jan 07, 2004 at 03:06:40PM +0100, Jens Axboe wrote:
    > > On Mon, Jan 05 2004, Matt Mackall wrote:
    > > > This is the fourth release of the -tiny kernel tree. The aim of this
    > > > tree is to collect patches that reduce kernel disk and memory
    > > > footprint as well as tools for working on small systems. Target users
    > > > are things like embedded systems, small or legacy desktop folks, and
    > > > handhelds.
    > > >
    > > > Latest release includes:
    > > > - various compile fixes for last release
    > > > - actually include Andi Kleen's bloat-o-meter this time
    > > > - optional mempool removal
    > >
    > > Your CONFIG_MEMPOOL is completely broken as you are no longer giving the
    > > same guarentees (you have no reserve at all). Might as well change it to
    > > CONFIG_DEADLOCK instead.
    > It's equivalent to a pool size of zero, yes, so deadlock odds are
    > significantly higher with some usage scenarios. I'll add a big fat
    > warning.

    Precisely. In most scenarios it makes deadlocks possible, where it was
    safe before (more below).

    > On the other hand, the existence of pre-allocated mempools can greatly
    > increase the likelihood of starvation, oom, and deadlock on the rest
    > of the system, especially as it becomes a greater percentage of the
    > total free memory on a small system. In other words, I had to cut this
    > corner to make running in 2M work with my config. When I merge
    > CONFIG_BLOCK, it'll be more generally useful.

    It needs to be carefulled tuned, definitely.

    > For the sake of our other readers, I'll point out that mempool doesn't
    > intrinisically reduce deadlock odds to zero unless we have a hard
    > limit on requests in flight that's strictly less than pool size.

    That's not true, depends entirely on usage. It's not a magic wand. And
    you don't need a hard limit, you only need progress guarentee. Typically
    just a single pre-allocated object can make you 100% deadlock free, if
    stacking is not involved. So for most cases, I think it would be much
    better if you just hard wired min_nr to 1, that would move you from 90%
    to 99% safe :-)

    Jens Axboe

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