On Wed, Aug 17, 2016 at 1:03 PM, Srividya Desireddy <srividya.dr@xxxxxxxxxxx> wrote: > This series of patches optimize the memory utilized by zswap for storing > the swapped out pages. > > Zswap is a cache which compresses the pages that are being swapped out > and stores them into a dynamically allocated RAM-based memory pool. > Experiments have shown that around 10-15% of pages stored in zswap are > duplicates which results in 10-12% more RAM required to store these > duplicate compressed pages. Around 10-20% of pages stored in zswap > are zero-filled pages, but these pages are handled as normal pages by > compressing and allocating memory in the pool. > > The following patch-set optimizes memory utilized by zswap by avoiding the > storage of duplicate pages and zero-filled pages in zswap compressed memory > pool. > > Patch 1/4: zswap: Share zpool memory of duplicate pages > This patch shares compressed pool memory of the duplicate pages. When a new > page is requested for swap-out to zswap; search for an identical page in > the pages already stored in zswap. If an identical page is found then share > the compressed page data of the identical page with the new page. This > avoids allocation of memory in the compressed pool for a duplicate page. > This feature is tested on devices with 1GB, 2GB and 3GB RAM by executing > performance test at low memory conditions. Around 15-20% of the pages > swapped are duplicate of the pages existing in zswap, resulting in 15% > saving of zswap memory pool when compared to the baseline version. > > Test Parameters Baseline With patch Improvement > Total RAM 955MB 955MB > Available RAM 254MB 269MB 15MB > Avg. App entry time 2.469sec 2.207sec 7% > Avg. App close time 1.151sec 1.085sec 6% > Apps launched in 1sec 5 12 7 > > There is little overhead in zswap store function due to the search > operation for finding duplicate pages. However, if duplicate page is > found it saves the compression and allocation time of the page. The average > overhead per zswap_frontswap_store() function call in the experimental > device is 9us. There is no overhead in case of zswap_frontswap_load() > operation. > > Patch 2/4: zswap: Enable/disable sharing of duplicate pages at runtime > This patch adds a module parameter to enable or disable the sharing of > duplicate zswap pages at runtime. > > Patch 3/4: zswap: Zero-filled pages handling > This patch checks if a page to be stored in zswap is a zero-filled page > (i.e. contents of the page are all zeros). If such page is found, > compression and allocation of memory for the compressed page is avoided > and instead the page is just marked as zero-filled page. > Although, compressed size of a zero-filled page using LZO compressor is > very less (52 bytes including zswap_header), this patch saves compression > and allocation time during store operation and decompression time during > zswap load operation for zero-filled pages. Experiments have shown that > around 10-20% of pages stored in zswap are zero-filled. Aren't zero-filled pages already handled by patch 1/4 as their contents match? So the overall memory saving is 52 bytes? - Pekka -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>