> Re-sending this in plain text format (Apologies) > > Hello, > > In pursuit of saving memory on Android, I started experimenting with > Kernel Same Page Merging(KSM). > Number of pages shared because of KSM is reported by > /sys/kernel/mm/pages_sharing. > Documentation/vm/ksm.txt explains this as: > > "pages_sharing - how many more sites are sharing them i.e. how much saved" > > After enabling KSM on Android device, this number was reported as 19666 pages. > Obvious optimization is to find out source of sharing and see if we > can avoid duplicate pages at first place. > In order to collect the data needed, It needed few > modifications(trace_printk) statement in mm/ksm.c. > Data should be collected from second cycle because that's when ksm > starts merging > pages. First KSM cycle is only used to calculate the checksum, pages > are added to > unstable tree and eventually moved to stable tree after this. > > After analyzing data from second KSM cycle, few things which stood out: > 1. In the same cycle, KSM can scan same page multiple times. Scanning > a page involves > comparing page with pages in stable tree, if no match is found > checksum is calculated. > From the look of it, it seems to be cpu intensive operation and > impacts dcache as well. > > 2. Same page which is already shared by multiple process can be > replaced by KSM page. > In this case, let say a particular page is mapped 24 times and is > replaced by KSM page then > eventually all 24 entries will point to KSM page. pages_sharing > will account for all 24 pages. > so pages _sharing does not actually report amount of memory saved. > From the above example actual > savings is one page. > > Both cases happen very often with Android because of its architecture > - Zygote spawning(fork) multiple > applications. To calculate actual savings, we should account for same > page(pfn)replaced by same KSM page only once. > In the case 2 example, page_sharing should account only one page. > After recalculating memory saving comes out to be 8602 pages (~34MB). > > I am trying to find out right solution to fix pages_sharing and > eventually optimize KSM to scan page > once even if it is mapped multiple times. > > Comments? > Correct Izik's email address.
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