On 17.03.22 03:03, CGEL wrote: > On Wed, Mar 16, 2022 at 03:56:23PM +0100, David Hildenbrand wrote: >> On 16.03.22 14:34, cgel.zte@xxxxxxxxx wrote: >>> From: Yang Yang <yang.yang29@xxxxxxxxxx> >>> >>> Delay accounting does not track the delay of ksm cow. When tasks >>> have many ksm pages, it may spend a amount of time waiting for ksm >>> cow. >>> >>> To get the impact of tasks in ksm cow, measure the delay when ksm >>> cow happens. This could help users to decide whether to user ksm >>> or not. >>> >>> Also update tools/accounting/getdelays.c: >>> >>> / # ./getdelays -dl -p 231 >>> print delayacct stats ON >>> listen forever >>> PID 231 >>> >>> CPU count real total virtual total delay total delay average >>> 6247 1859000000 2154070021 1674255063 0.268ms >>> IO count delay total delay average >>> 0 0 0ms >>> SWAP count delay total delay average >>> 0 0 0ms >>> RECLAIM count delay total delay average >>> 0 0 0ms >>> THRASHING count delay total delay average >>> 0 0 0ms >>> KSM count delay total delay average >>> 3635 271567604 0ms >>> >> >> TBH I'm not sure how particularly helpful this is and if we want this. >> > Thanks for replying. > > Users may use ksm by calling madvise(, , MADV_MERGEABLE) when they want > save memory, it's a tradeoff by suffering delay on ksm cow. Users can > get to know how much memory ksm saved by reading > /sys/kernel/mm/ksm/pages_sharing, but they don't know what the costs of > ksm cow delay, and this is important of some delay sensitive tasks. If > users know both saved memory and ksm cow delay, they could better use > madvise(, , MADV_MERGEABLE). But that happens after the effects, no? IOW a user already called madvise(, , MADV_MERGEABLE) and then gets the results. So how is this interface useful except for somebody writing an application and simply being able to benchmark it with vs. without MADV_MERGEABLE? -- Thanks, David / dhildenb
