Andrew Morton wrote:
On Thu, 18 Oct 2007 16:15:31 -0400
Marcelo Tosatti <[email protected]> wrote:
Hi,
AIX contains the SIGDANGER signal to notify applications to free up some
unused cached memory:
http://www.ussg.iu.edu/hypermail/linux/kernel/0007.0/0901.html
There have been a few discussions on implementing such an idea on Linux,
but nothing concrete has been achieved.
On the kernel side Rik suggested two notification points: "about to
swap" (for desktop scenarios) and "about to OOM" (for embedded-like
scenarios).
With that assumption in mind it would be necessary to either have two
special devices for notification, or somehow indicate both events
through the same file descriptor.
Comments are more than welcome.
Martin was talking about some mad scheme wherin you'd create a bunch of
pseudo files (say, /proc/foo/0, /proc/foo/1, ..., /proc/foo/9) and each one
would become "ready" when the MM scanning priority reaches 10%, 20%, ...
100%.
Obviously there would need to be a lot of abstraction to unhook a permanent
userspace feature from a transient kernel implementation, but the basic
idea is that a process which wants to know when the VM is getting into the
orange zone would select() on the file "7" and a process which wants to
know when the VM is getting into the red zone would select on file "9".
It get more complicated with NUMA memory nodes and cgroup memory
controllers.
We ended up not doing that, but making a scanner that saw what
percentage of the LRU was touched in the last n seconds, and
printing that to userspace to deal with.
Turns out priority is a horrible metric to use for this - it
stays at default for ages, then falls off a cliff far too
quickly to react to.
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