User-space software suspend [LWN.net]

User-space software suspend

Posted Oct 6, 2005 17:36 UTC (Thu) by peschmae (guest, #32292)
In reply to: User-space software suspend by zblaxell
Parent article: User-space software suspend

> I do like the fact that swsusp2 resumes with caches and buffers intact.
> If I wanted to wait while the system painfully restored this data one 4K
> page fault from swap at a time, I might as well reboot--it could actually
> be faster.

Me too. But on my machine (laptop - harddisk is slow) rebooting would still be slower ;-)

> On the other hand, I generally like to run a small application before
> suspending, which allocates memory until a few hundred pages are swapped
> (it is a loop of malloc() and reading paging statistics out of /proc),
> then exits. This dumps out some of the more useless 400MB or so of caches
> on my system, and cuts resume time in half (it does add a second or two
> to suspend), without the extreme pain of having to swap _everything_ back
> in on resume.

Isn't that exactly what the # ImageSizeLimit 200 item in hibernate.conf (or the /proc/software_suspend/image_size_limit respectively) are there for?

Does your way of doing the more or less same thing have an advantage over that? (Faster maybe?)

> I'm not sure what benefit there is in pushing too much of the suspend and
> resume functions into user space.

I agree here. Because it still seems to need very much code in the kernel - only a minimal part is user space application.
And I don't really like it if the kernel depends on user space apps to boot - only makes for trouble (the tool has to be on the initrd (I don't like initrds anyway - at least not for my custom built kernels))

Peschmä

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User-space software suspend

Posted Oct 6, 2005 19:11 UTC (Thu) by zblaxell (subscriber, #26385) [Link]

Normally suspend2 writes all non-free pages (including clean cache pages and cached swap pages). This is a bit annoying for me, since 90% of the time I use less than 40% of my laptop's memory, but I have to wait for the other 60% of the RAM to be read and written at suspend and resume time.

ImageSizeLimit is an upper bound on the image size. If the image would be larger than this, then there is a pre-suspend forcing of pages--dirty or not--to disk. If the value is not dynamically chosen, it is inefficient--too high, and unnecessary pages are written in the suspend image; too low, and suspend and resume time is significantly increased since a bunch of stuff has to be swapped out before suspend and back in after resume, and page for page the swapper is much slower than Suspend2's image writer. Dynamically choosing the value is apparently non-trivial...at least I tried to do it for a while, then gave up.

My application forces all the clean pages (600MB as I write this) to go away, without losing active program text pages or forcing dirty pages to swap. It stops as soon as there are more than 100 pages written to swap since the program started running, so it does not significantly extend the suspend time (a few hundred pages are swapped before the application notices and exits, which does take a second or so).

This approach doesn't need prior configuration--it automatically discovers just how much RAM can be cheaply freed by allocating as much as the system can spare without swapping, then it exits and leaves thousands of free pages.

Without all the extra pages, the suspend image is much smaller, so suspend and resume are faster. Since only a few dirty or active pages were actually swapped, it doesn't noticeably slow down the machine after resume (there is more overhead when xscreensaver wakes up after noticing the wall clock time jumping well past the inactivity threshold, than there is from post-resume swapping ;-).

User-space software suspend

Posted Oct 30, 2005 1:51 UTC (Sun) by NinjaSeg (guest, #33460) [Link]

Errr, care to share it with us?

User-space software suspend

Posted Nov 4, 2005 0:43 UTC (Fri) by zblaxell (subscriber, #26385) [Link]

#!/usr/bin/perl -w
use strict;
use Time::HiRes qw(time);

sub swapfree {
open(PROC, "/proc/meminfo") or die "open: /proc/meminfo: $!";
my ($swapfree) = grep(/^SwapFree:/, <PROC>);
close(PROC);
$swapfree =~ s/\D+//gos;
print STDERR "swapfree=$swapfree\n";
return $swapfree;
}

my $last_swapfree = swapfree;
my @blobs;

my $count = 0;
my $total = 0;

my $start_time = time;

while ($last_swapfree <= (my $new_swapfree = swapfree)) {
++$count;
push(@blobs, ('.' x (1048576 * $count)));
$total += $count;
print STDERR "${total}M allocated\n";
$last_swapfree = $new_swapfree;
}
system("ps m $$");
print STDERR time - $start_time, " seconds\n";