On 23/09/2024 00:57, Barry Song wrote: > On Thu, Sep 5, 2024 at 7:36 AM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: >> >> [..] >>>> >>>> On the other hand, if you read the code of zRAM, you will find zRAM has >>>> exactly the same mechanism as zeromap but zRAM can even do more >>>> by same_pages filled. since zRAM does the job in swapfile layer, there >>>> is no this kind of consistency issue like zeromap. >>>> >>>> So I feel for zRAM case, we don't need zeromap at all as there are duplicated >>>> efforts while I really appreciate your job which can benefit all swapfiles. >>>> i mean, zRAM has the ability to check "zero"(and also non-zero but same >>>> content). after zeromap checks zeromap, zRAM will check again: >>>> >>> >>> Yes, so there is a reason for having the zeromap patches, which I have outlined >>> in the coverletter. >>> >>> https://lore.kernel.org/all/20240627105730.3110705-1-usamaarif642@xxxxxxxxx/ >>> >>> There are usecases where zswap/zram might not be used in production. >>> We can reduce I/O and flash wear in those cases by a large amount. >>> >>> Also running in Meta production, we found that the number of non-zero filled >>> complete pages were less than 1%, so essentially its only the zero-filled pages >>> that matter. >>> >>> I believe after zeromap, it might be a good idea to remove the page_same_filled >>> check from zram code? Its not really a problem if its kept as well as I dont >>> believe any zero-filled pages should reach zram_write_page? >> >> I brought this up before and Sergey pointed out that zram is sometimes >> used as a block device without swap, and that use case would benefit >> from having this handling in zram. That being said, I have no idea how >> many people care about this specific scenario. > > Hi Usama/Yosry, > > We successfully gathered page_same_filled data for zram on Android. > Interestingly, > our findings differ from yours on zswap. > > Hailong discovered that around 85-86% of the page_same_filled data > consists of zeros, > while about 15% are non-zero. We suspect that on Android or similar > systems, some > graphics or media data might be duplicated at times, such as a red > block displayed > on the screen. > > Does this suggest that page_same_filled could still provide some > benefits in zram > cases? Hi Barry, Thanks for the data, its very interesting to know this from mobile side. Eventhough its not 99% that I observed, I do feel 85% is still quite high. The 2 main reasons for the patcheset to store zero pages to be swapped out in a bitmap were for applications that use swap but not zswap/zram (reducing I/O and flash wear), and simplifying zswap code. It also meant fewer zswap_entry structs in memory which would offset the memory usage by bitmap. Yosry mentioned that Sergey pointed out that zram is sometimes used as a block device without swap, and that use case would benefit from having this handling in zram. Will that case also not go through swap_writepage and therefore be takencare of by swap_info_struct->zeromap? Also just curious if there are cases in mobile where only swap is used, but not zswap/zram? I think even with 85% zero-filled pages, we could get rid of page_same_filled especially if the zram without swap case is handled by swap_info_struct->zeromap. But don't have strong preference. Thanks, Usama > > Thanks > Barry
