Could you please review this patch series and update if any corrections are needed in the patch-set. -Srividya On Fri, Aug 19, 2016 at 11:04 AM, Srividya Desireddy wrote: > On 17 August 2016 at 18:08, Pekka Enberg wrote: >> On Wed, Aug 17, 2016 at 1:03 PM, Srividya Desireddy >> 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 > > Thanks for the quick reply. > > Zero-filled pages can also be handled by patch 1/4. It performs > searching of a duplicate page among existing stored pages in zswap. > Its been observed that average search time to identify duplicate zero > filled pages(using patch 1/4) is almost thrice compared to checking > all pages for zero-filled. > > Also, in case of patch 1/4, the zswap_frontswap_load() operation requires > the compressed zero-filled page to be decompressed. zswap_frontswap_load() > function in patch 3/4 just fills the page with zeros while loading a > zero-filled page and is faster than decompression. > > - Srividya
