On 2013-10-26 20:37, Leszek Lesner wrote: > As some of you know there were some discussions on this mailinglist > regarding zram and vm.swappiness. > In this mail I want to write down all I know and learned about zram, > vm.swappiness and its cooperation. > This is intend as a help and a starting point for discussions for the > next release of Lubuntu which will be an LTS. > > I have to thank Mélodie for the discussions about vm.swappiness which > made me dig deeper and deeper into it. > And I have to say I was wrong at least partly :P > > So before talking about ZRAM a small comprehensive introduction what it is. > ZRAM is a compressed virtual swap device that lives in RAM. With this it > is possible to use and store more memory > that is physically available. It is used by Lubuntu in 13.10 by default. > > What is vm.swappiness ? That is a far more complicated thing to explain. > Simply speaking it controls the balance between swapping out runtime > memory, > as opposed to dropping pages from the system page cache. It can have a > value from 0 to 100. > A lower value means that it will try not to swap out page cache but keep > it in memory. > A high value means try to more aggressively swap out unused page memory. > > A discussion was started if Lubuntu should lower down the vm.swappiness > value or even set it down to 0 > to improve system performance and responsiveness. > In my initial thought that I could give a universal answer on this topic > but I was wrong. > There a several scenarios we need to take a look at to see that there is > not an universal answer on this. > > == The classical situation (before 13.10): == > * RAM + SWAP (on HDD) > > The RAM is backed up by a SWAP partition or SWAP file which is stored on > a harddrive or ssd. > As we all know disk i/o is a lot slower than RAM. On older machines and > slower disks this could lead to noticable > lags while swapping. But also on normal machines it is noticable. So for > this scenario it would be nice to tell the > kernel to avoid swapping as much as possible to avoid this bottleneck. > A low vm.swappiness or even 0 is a good setting here as the kernel keeps > filling the ram until it is almost full and then > starts swapping out which could make the system laggy for a few seconds > and while writing data to swap. So there is an unavoidable > bottleneck here but there is also the possibility that with enough ram > and a low memory footprint of the apps and desktop that > the user will not reach the point of swapping. > > == The new modern situation (with 13.10¹): == > * RAM + ZRAM > > The RAM is backed up by a SWAP partition which is stored on a virtual > compressed ram device. Under Lubuntu > 13.10 this device can use half of the physically available memory. > Writing and reading to and from RAM is a hell lot faster than to disk > (even to an SSD). This means there is no bootlneck > when the system swaps out to ZRAM with its default swappiness settings. > Setting the swappiness lower here would make a change but > a very minor only (noticable only via benchmarks). And lowering it here > might lead to a faster reaching of the second > bottleneck. So avoiding swapping and letting the RAM be filled and > swapping late especially when RAM is very full will slow down the > system and produces noticable lags. Those lags might be even harder in > this case because the pages need to be compressed into RAM and > when new pages need to be placed into RAM at the same time this might > lead to a long cpu 100% compressing decompressing moving loop. > Letting the swappiness at default (60) or even set it higher to 100 > would help avoid this bottleneck as swapping early would avoid > RAM running full too fast. > Whats the downside ? > The CPU has to compress and decompress more often (don't worry even a > Pentium 3 is very fast at it) and this causes more wake up times > for the CPU. (perhaps a downside for those who want to save every little > watt of there notebook battery) > > == The combination of both situations : == > * RAM + ZRAM + SWAP (on HDD) > > The RAM is backed up by ZRAM and a SWAP partition/file. This is an usual > setting if you have very low RAM (like 256/512MB or 1 GB). > By default ZRAM is as Swap device has a higher priority than the swap > partition/file on hd. The swapping mechanism now works very clevely > and swaps out first and by default early to zram first until its full. > Then it will start swapping to hdd. > This will lead us to our bottleneck nr.1 again but in contrast to the > classical situation (scenario 1) it will get there a lot later :) > So is there a possibility to avoid this bottleneck by changing the > swappiness ? No not really. > Swappiness low would lead too faster filling in memory which will lead > (especially on low memory systems) lead to bootlneck 2 which tends > to make the system a lot laggier than bottleneck 1. > Setting swappiness higher here would be also only work until ZRAM is > filled and than would lead to bottleneck 1 again. > So overall I would recommend the default swappiness value here which > tends to keep the system in a good balance between bottleneck 1 and 2. > > So all in all when you see this scenarios I have to say that the default > value of swappiness 60 makes the most sense as the default value > of lubuntu. Changing it for the next release is something I would not > recommend. > > Seeing this one could ask wouldn't it be good to directly compress any > data written to RAM ? > And yes this is a possibility in the future with zcache (which is > basically a backend for cleancache which is a whole framework around it). > Zcache compresses every page cache written to RAM which will lead to > more available free memory. > And yes it is even possible to combine ZCache (Page Cache[Disk Cache and > so on]) and ZRAM (Application memory) which > could lead to even more available free RAM and might even make it > possible to completely get rid of SWAP partitions/files on hdd. > In the making is also frontswap which is basically speaking zram but > without the necessity to give it a fixed size. So it can dynamically > grow. > > So the future looks very promising when it comes to low ram management > and we are on a good way. > Thank you for an interesting write-up about zRAM and swappiness :-) Best regards Nio
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