Cc: Pavel Machek <pavel@...>, Peter Zijlstra <a.p.zijlstra@...>, <linux-mm@...>, <linux-kernel@...>, <akpm@...>, <dkegel@...>, David Miller <davem@...>, Nick Piggin <npiggin@...>
On Friday 26 October 2007 10:55, Christoph Lameter wrote:
If it does, it is a bug in the reserve accounting. That said, I still
agree with you that per-node reserve is a desirable goal for numa. I
would just like to be clear that it is not necessary, even for numa,
just nice. By all means somebody should be hacking on a numa feature
for per-node emergency reserves, but as far as fixing the immediate,
serious kernel block IO deadlocks goes, it does not matter.
Pavel, I do not agree that efficiency is unimportant on the
under-pressure path. I do not even like to call that the "emergency"
path, because under heavy load it is normal for a machine to spend a
significant fraction of its time in that state. However, the
efficiency goal there does not need to be quite the same as normal
mode.
To illustrate, I would expect to see something like 95% of normal block
IO performance on a numa machine in the case that "emergency" (aka
memalloc memory) is allocated globally instead of locally, thus paying
a (modest compared to the disk transfer itself) penalty for transfer of
disk data over the numa interconnect. 95% of normal throughput on the
block IO path is not a problem: if the machine spends 5% of its time on
the "emergency" (aka memalloc) path, then overall efficiency will be
95% * 95% = 99.75%.
Moral of this story: let's get the memory recursion fixes done in the
most obviously correct way and not get distracted by illusory
efficiency requirements for numa, that do not have a big bottom line
impact.
I'm glad to see everybody still interested in these problems. Though we
have been a little quiet on this issue over here for a while, it does
not mean that progress has stopped. In fact, we are testing our
solutions more heavily than ever, and getting closer to a solution that
not only works solidly, but that should enable mass deletion of the
whole creaky notion of dirty page limits in favor of nice, tight
per-device control of in flight write traffic as I have described
previously.
Regards,
Daniel
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