On Wed, 5 Jun 2024 16:58:11 -0700 Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > On Wed, Jun 5, 2024 at 4:53 PM Yu Zhao <yuzhao@xxxxxxxxxx> wrote: > > > > On Wed, Jun 5, 2024 at 5:42 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > > > On Wed, Jun 5, 2024 at 4:04 PM Erhard Furtner <erhard_f@xxxxxxxxxxx> wrote: > > > > > > > > On Tue, 4 Jun 2024 20:03:27 -0700 > > > > Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > > > > > > Could you check if the attached patch helps? It basically changes the > > > > > number of zpools from 32 to min(32, nr_cpus). > > > > > > > > Thanks! The patch does not fix the issue but it helps. > > > > > > > > Means I still get to see the 'kswapd0: page allocation failure' in the dmesg, a 'stress-ng-vm: page allocation failure' later on, another kswapd0 error later on, etc. _but_ the machine keeps running the workload, stays usable via VNC and I get no hard crash any longer. > > > > > > > > Without patch kswapd0 error and hard crash (need to power-cycle) <3min. With patch several kswapd0 errors but running for 2 hrs now. I double checked this to be sure. > > > > > > Thanks for trying this out. This is interesting, so even two zpools is > > > too much fragmentation for your use case. > > > > Now I'm a little bit skeptical that the problem is due to fragmentation. > > > > > I think there are multiple ways to go forward here: > > > (a) Make the number of zpools a config option, leave the default as > > > 32, but allow special use cases to set it to 1 or similar. This is > > > probably not preferable because it is not clear to users how to set > > > it, but the idea is that no one will have to set it except special use > > > cases such as Erhard's (who will want to set it to 1 in this case). > > > > > > (b) Make the number of zpools scale linearly with the number of CPUs. > > > Maybe something like nr_cpus/4 or nr_cpus/8. The problem with this > > > approach is that with a large number of CPUs, too many zpools will > > > start having diminishing returns. Fragmentation will keep increasing, > > > while the scalability/concurrency gains will diminish. > > > > > > (c) Make the number of zpools scale logarithmically with the number of > > > CPUs. Maybe something like 4log2(nr_cpus). This will keep the number > > > of zpools from increasing too much and close to the status quo. The > > > problem is that at a small number of CPUs (e.g. 2), 4log2(nr_cpus) > > > will actually give a nr_zpools > nr_cpus. So we will need to come up > > > with a more fancy magic equation (e.g. 4log2(nr_cpus/4)). > > > > > > (d) Make the number of zpools scale linearly with memory. This makes > > > more sense than scaling with CPUs because increasing the number of > > > zpools increases fragmentation, so it makes sense to limit it by the > > > available memory. This is also more consistent with other magic > > > numbers we have (e.g. SWAP_ADDRESS_SPACE_SHIFT). > > > > > > The problem is that unlike zswap trees, the zswap pool is not > > > connected to the swapfile size, so we don't have an indication for how > > > much memory will be in the zswap pool. We can scale the number of > > > zpools with the entire memory on the machine during boot, but this > > > seems like it would be difficult to figure out, and will not take into > > > consideration memory hotplugging and the zswap global limit changing. > > > > > > (e) A creative mix of the above. > > > > > > (f) Something else (probably simpler). > > > > > > I am personally leaning toward (c), but I want to hear the opinions of > > > other people here. Yu, Vlastimil, Johannes, Nhat? Anyone else? > > > > I double checked that commit and didn't find anything wrong. If we are > > all in the mood of getting to the bottom, can we try using only 1 > > zpool while there are 2 available? I.e., > > Erhard, do you mind checking if Yu's diff below to use a single zpool > fixes the problem completely? There is also an attached patch that > does the same thing if this is easier to apply for you. No, setting ZSWAP_NR_ZPOOLS to 1 does not fix the problem unfortunately (that being the only patch applied on v6.10-rc2). Trying to alter the lowmem and virtual mem limits next as Michael suggested. Regards, Erhard
