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 <semenzato@xxxxxxxxxx> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>