On Sun, Jun 07, 2020 at 05:25:15PM -0600, Chris Murphy wrote: >> This is not generally true, only if RAM gets so tight that applications >> start competing for swap. >> This is why I've proposed test cases testing exactly that, as for >> the case of persistent swap I'd expect the outcome to be a clear win for >> disk swap. (Although this can in some cases also be seen as bug, as this >> would be applications not really using the allocated space) > > I don't follow this. Where are the proposed test cases? And also in > what case are you saying disk swap is a clear win? I was referencing the testcases from the email before that, but your webkitgtk compile might also work for that. What I described as persistent swap is stuff that gets swapped out and not swapped back in for hours or days. >> Until about 95% mem usage I'd expect the disk swap case to win, as it >> should behave the same as no swap (with matching swappiness values) > > Why would disk based swap win? In this example, where there's been no > page outs, the zram device isn't using any memory. Again, it is not a > preallocation. Yes, its a quite boring example, but I've included it for completeness as a border case. This is just the few megabytes it needs preallocated, whilst swap is not in use at all. >> At 150% memory usage assuming a 2:1 compression ratio this would mean: >> - disk swap: >> has to write 4G to disk initially, and for reading swap another 4G >> (12G total traffic - 4G initial, 4G swapping out and 4G swapping in) >> - zram, assuming 4G zram swap: >> has to write 8G to zram initially, and for reading the data swap 16G >> (24G total traffic - 8G initial, 8G swapping out and 8G swapping in) > > swap contains anonymous pages, so I'm not sure what you mean by > initial. Whether these pages are internet or typed in or come from > persistent storage - it's a wash between disk or zram swap so it can > be ignored. I was calculating it from the viewpoint of data, e.g. paging out a certain amount of data, and paging it in again. "Initial" would be the amount of data when paging in. What is definitely different is that I thought of 1 or 2 processes eating away memory, but not of many thrashing swap. For those it is definitely not possible to recover from it once thrashing has started. > Also I don't understand any of your math,how you start with a 4G zram > swap but have 8G. I think you're confused. The cap of 4GiB is the > device size. The actual amount of RAM it uses will be less due to > compression. The zram device size is not the amount of memory used. > And in no case is there a preallocation of memory unless the zram > device is used. It is easy to get confused, by the way. That was my > default state for days upon first stumbling on this. I assumed a 2:1 compression rate, so the zram swap holds 8G of data in a 4G zram device. I've calculated with filling the zram device to the max, so it will use the full 4G. (the 4G limit was arbitrarily chosen) > This task only succeeds with ~12+G of disk based swap. Which is just > not realistic. It's a clearly overcommitted and thus contrived test. This sounds like it's just failing earlier. But it's still a test case. > But I love it and hate it at the same time. More realistic is to not > use defaults, and set the number of jobs manually to 6. And in this > case, zram based swap consistently beats disk based swap. > Which makes sense because pretty much all of the inactive pages are > going to be needed at some point by the compile or they are dropped. > Following the compile there aren't a lot of inactive pages left, and > I'm not sure they're even related to the compile at all. Especially for a compile those pages are needed quite soon, so thrashing occurs earlier too. For this it makes a lot of sense that zram is a big benefit for it. When I reached the memory limit my usecase was usually having chrome and firefox open, with firefox having about 500 open tabs, so most of the data could stay in swap until I triggered swap in, which is very different from a compiling. > Even under manual control we've got examples of the GUI becoming > completely stuck. Long threads in devel@ based on this Workstation > working group issue - with the same name. So just search archives for > interactivity. Or maybe webkitgtk. I'm afraid I've read most of those, I usually read all mails to devel@. So far it seemed mostly like exceptions, but it might also be a specific configuration on my systems and this issue is more widespread. > earlyoom will kill in such a case even if you can't. It's configurable > and intentionally simplistic, based on memory and swap free > percentage. I don't have any experience with it, as I use the time from slowdown until OOM to try to manage the issue myself, usually successful. But as mentioned above, I might have a specialized usecase, so my experience might not reflect the average users' experience. All the best, David
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