Minchan:

I tried your suggestion to move the call to wake_all_kswapd from after
"restart:" to after "rebalance:".  The behavior is still similar, but
slightly improved.  Here's what I see.

Allocating as fast as I can: 1.5 GB of the 3 GB of zram swap are used,
then OOM kills happen, and the system ends up with 1 GB swap used, 2
unused.

Allocating 10 MB/s: some kills happen when only 1 to 1.5 GB are used,
and continue happening while swap fills up.  Eventually swap fills up
completely.  This is better than before (could not go past about 1 GB
of swap used), but there are too many kills too early.  I would like
to see no OOM kills until swap is full or almost full.

Allocating 20 MB/s: almost as good as with 10 MB/s, but more kills
happen earlier, and not all swap space is used (400 MB free at the
end).

This is with 200 processes using 20 MB each, and 2:1 compression ratio.

So it looks like kswapd is still not aggressive enough in pushing
pages out.  What's the best way of changing that?  Play around with
the watermarks?

Incidentally, I also tried removing the min_filelist_kbytes hacky
patch, but, as usual, the system thrashes so badly that it's
impossible to complete any experiment.  I set it to a lower minimum
amount of free file pages, 10 MB instead of the 50 MB which we use
normally, and I could run with some thrashing, but I got the same
results.

Thanks!
Luigi


On Wed, Nov 28, 2012 at 4:31 PM, Luigi Semenzato [off-list ref] wrote:

I am beginning to understand why zram appears to work fine on our x86
systems but not on our ARM systems.  The bottom line is that swapping
doesn't work as I would expect when allocation is "too fast".

In one of my tests, opening 50 tabs simultaneously in a Chrome browser
on devices with 2 GB of RAM and a zram-disk of 3 GB (uncompressed), I
was observing that on the x86 device all of the zram swap space was
used before OOM kills happened, but on the ARM device I would see OOM
kills when only about 1 GB (out of 3) was swapped out.

I wrote a simple program to understand this behavior.  The program
(called "hog") allocates memory and fills it with a mix of
incompressible data (from /dev/urandom) and highly compressible data
(1's, just to avoid zero pages) in a given ratio.  The memory is never
touched again.

It turns out that if I don't limit the allocation speed, I see
premature OOM kills also on the x86 device.  If I limit the allocation
to 10 MB/s, the premature OOM kills stop happening on the x86 device,
but still happen on the ARM device.  If I further limit the allocation
speed to 5 Mb/s, the premature OOM kills disappear also from the ARM
device.

I have noticed a few time constants in the MM whose value is not well
explained, and I am wondering if the code is tuned for some ideal
system that doesn't behave like ours (considering, for instance, that
zram is much faster than swapping to a disk device, but it also uses
more CPU).  If this is plausible, I am wondering if anybody has
suggestions for changes that I could try out to obtain a better
behavior with a higher allocation speed.

Thanks!
Luigi

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