On Wed, Aug 17, 2016 at 1:18 PM, Srividya Desireddy <[email protected]> wrote: >> This patch adds a check in zswap_frontswap_store() to identify zero-filled >> page before compression of the page. If the page is a zero-filled page, set >> zswap_entry.zeroflag and skip the compression of the page and alloction >> of memory in zpool. In zswap_frontswap_load(), check if the zeroflag is >> set for the page in zswap_entry. If the flag is set, memset the page with >> zero. This saves the decompression time during load. >> >> The overall overhead caused due to zero-filled page check is very minimal >> when compared to the time saved by avoiding compression and allocation in >> case of zero-filled pages. The load time of a zero-filled page is reduced >> by 80% when compared to baseline.
On Wed, Aug 17, 2016 at 3:25 PM, Pekka Enberg <[email protected]> wrote: > AFAICT, that's an overall improvement only if there are a lot of > zero-filled pages because it's just overhead for pages that we *need* > to compress, no? So I suppose the question is, are there a lot of > zero-filled pages that we need to swap and why is that the case? I suppose reading your cover letter would have been helpful before sending out my email: "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." But I still don't understand why we have zero-filled pages that we are swapping out. - Pekka
