Minchan Kim <minchan@xxxxxxxxxx> writes: > On Fri, Sep 15, 2017 at 11:15:08AM +0800, Huang, Ying wrote: >> Minchan Kim <minchan@xxxxxxxxxx> writes: >> >> > On Thu, Sep 14, 2017 at 08:01:30PM +0800, Huang, Ying wrote: >> >> Minchan Kim <minchan@xxxxxxxxxx> writes: >> >> >> >> > On Wed, Sep 13, 2017 at 02:02:29PM -0700, Andrew Morton wrote: >> >> >> On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minchan@xxxxxxxxxx> wrote: >> >> >> >> >> >> > Every zram users like low-end android device has used 0 page-cluster >> >> >> > to disable swap readahead because it has no seek cost and works as >> >> >> > synchronous IO operation so if we do readahead multiple pages, >> >> >> > swap falut latency would be (4K * readahead window size). IOW, >> >> >> > readahead is meaningful only if it doesn't bother faulted page's >> >> >> > latency. >> >> >> > >> >> >> > However, this patch introduces additional knob /sys/kernel/mm/swap/ >> >> >> > vma_ra_max_order as well as page-cluster. It means existing users >> >> >> > has used disabled swap readahead doesn't work until they should be >> >> >> > aware of new knob and modification of their script/code to disable >> >> >> > vma_ra_max_order as well as page-cluster. >> >> >> > >> >> >> > I say it's a *regression* and wanted to fix it but Huang's opinion >> >> >> > is that it's not a functional regression so userspace should be fixed >> >> >> > by themselves. >> >> >> > Please look into detail of discussion in >> >> >> > http://lkml.kernel.org/r/%3C1505183833-4739-4-git-send-email-minchan@xxxxxxxxxx%3E >> >> >> >> >> >> hm, tricky problem. I do agree that linking the physical and virtual >> >> >> readahead schemes in the proposed fashion is unfortunate. I also agree >> >> >> that breaking existing setups (a bit) is also unfortunate. >> >> >> >> >> >> Would it help if, when page-cluster is written to zero, we do >> >> >> >> >> >> printk_once("physical readahead disabled, virtual readahead still >> >> >> enabled. Disable virtual readhead via >> >> >> /sys/kernel/mm/swap/vma_ra_max_order"). >> >> >> >> >> >> Or something like that. It's pretty lame, but it should help alert the >> >> >> zram-readahead-disabling people to the issue? >> >> > >> >> > It was my last resort. If we cannot find other ways after all, yes, it would >> >> > be a minimum we should do. But it still breaks users don't/can't read/modify >> >> > alert and program. >> >> > >> >> > How about this? >> >> > >> >> > Can't we make vma-based readahead config option? >> >> > With that, users who no interest on readahead don't enable vma-based >> >> > readahead. In this case, page-cluster works as expected "disable readahead >> >> > completely" so it doesn't break anything. >> >> >> >> Now. Users can choose between VMA based readahead and original >> >> readahead via a knob as follow at runtime, >> >> >> >> /sys/kernel/mm/swap/vma_ra_enabled >> > >> > It's not a config option and is enabled by default. IOW, it's under the radar >> > so current users cannot notice it. That's why we want to emit big fat warnning. >> > when old user set 0 to page-cluster. However, as Andrew said, it's lame. >> > >> > If we make it config option, product maker/kernel upgrade user can have >> > a chance to notice and read description so they could be aware of two weird >> > knobs and help to solve the problem in advance without printk_once warn. >> > If user has no interest about swap-readahead or skip the new config option >> > by mistake, it works physcial readahead which means no regression. >> >> I am OK to make it config option. But I think VMA based swap readahead >> should be enabled by default. Because per my understanding, default >> option should be set for most common desktop users. And VMA based swap >> readahead should benefit them. People needs to turn off swap readahead >> is some special users, the original swap readahead default configuration >> isn't for them too. > > Okay. I don't care either one is default if it is a config option. > It still gives a chance to notice a new algorithm so users can decide it > It is absolutely better than silent regressoin and printk tric. > Please add more description about those parallel two readahead algorithms > in somewhere(e.g., vm.txt) so he can understand the situation exactly and > can handle both tunable knobs at the same time. Sure. Best Regards, Huang, Ying -- 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>
