On Thu, Sep 14, 2017 at 08:53:04AM +0800, Huang, Ying wrote: > Hi, Andrew, > > Andrew Morton <a...@linux-foundation.org> writes: > > > On Wed, 13 Sep 2017 10:40:19 +0900 Minchan Kim <minc...@kernel.org> 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-minc...@kernel.org%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? > > This sounds good for me. > > Hi, Minchan, what do you think about this? I think for low-end android > device, the end-user may have no opportunity to upgrade to the latest > kernel, the device vendor should care about this. For desktop users, > the warning proposed by Andrew may help to remind them for the new knob.
Yes, it would be option. At least, we should alert to the user to make a chance to fix. However, can't we make vma-based readahead new config option? Please look at the detail in my reply of andrew. With that, there is no regression with current users and as a bonus, user can measure both algorithm with their real workload with both algorithm rather than artificial benchmark. I think recency vs spartial locality would have each pros and cons so that kind soft landing would be safer option rather than sudden replacing. After a while, we can set new algorithm as default.
