On Fri, 30 Sep 2022 02:00:32 +0000 xu xin <xu.xin.sc@xxxxxxxxx> wrote: > >> On 29.09.22 12:42, Claudio Imbrenda wrote: > >> > On Thu, 29 Sep 2022 02:52:06 +0000 > >> > xu.xin.sc@xxxxxxxxx wrote: > >> > > >> >> From: xu xin <xu.xin16@xxxxxxxxxx> > >> >> > >> >> Before enabling use_zero_pages by setting /sys/kernel/mm/ksm/ > >> >> use_zero_pages to 1, pages_sharing of KSM is basically accurate. But > >> >> after enabling use_zero_pages, all empty pages that are merged with > >> >> kernel zero page are not counted in pages_sharing or pages_shared. > >> > > >> > that's because those pages are not shared between different processes. > >> > >> They are probably the most shared pages between processes in the kernel. > > > >shared from the kernel, though, not from other processes (that's what I > >meant) > > > >> They are simply not KSM pages, that's what makes accounting tricky here. > > > >exactly. and those pages get shared all the time even without KSM, so > >why care about those now? > > > >does it make a difference why a page is a zero page? > > WI's necessary to show these sharing zeros pages. Because: > > 1) Turning on/off use_zero_pages shouldn't make it so not transparent with the > sharing zero pages. When administrators enable KSM and turn on use_zero_pages, > if much memory increases due to zero pages sharing but they don't know the > reasons compared to turning off use_zero_pages, isn't it confusing? I'm not sure I understand what you mean here > > 2) If no need to let users know how many full-zero-filled pages are merged by KSM > due to use_zero_pages, then also no need to show pages_sharing and pages_shared that's not true. showing pages_sharing and pages_shared helps understand how much memory would be needed for unsharing everything. when pages filled with zeroes are replaced with zero-pages, those will not get unshared. there is no way to know if those were actual zero-pages from the beginning, or if they were pages full of zeroes that have been replaced by KSM. > to users. Besides, the description of pages_sharing in Documentation is wrong and > MISLEADING when enabling use_zero_pages. I guess I can fix the documentation > > 3) As David supposes, it also help for estimating memory demands when each and every > shared page could get unshared. the current statistics show exactly how much memory would be needed if everything would get unshared. zero-pages would not get unshared. > > > >> > >> > > >> >> That is because the rmap_items of these ksm zero pages are not > >> >> appended to The Stable Tree of KSM. > >> >> > >> >> We need to add the count of empty pages to let users know how many empty > >> >> pages are merged with kernel zero page(s). > >> > > >> > why? > >> > > >> > do you need to know how many untouched zero pages a process has? > >> > > >> > does it make a difference if the zero page is really untouched or if it > >> > was touched in the past but it is now zero? > >> > >> I'd also like to understand the rationale. Is it about estimating memory > >> demands when each and every shared page could get unshared? > >> > >
