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KS2012: memcg/mm: Moving zcache towards the mainlineKS2012: memcg/mm: Moving zcache towards the mainlinePosted Sep 20, 2012 10:35 UTC (Thu) by Jonno (subscriber, #49613)In reply to: KS2012: memcg/mm: Moving zcache towards the mainline by djm1021 Parent article: KS2012: memcg/mm: Moving zcache towards the mainline
Why are pages decompressed as part of moving the page to disk, rather than as part of swapping in it to memory again? I can see no benefit of not storing the already compressed page on disk, deferring the decompression to when the content is actually needed (if ever).
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KS2012: memcg/mm: Moving zcache towards the mainline Posted Sep 20, 2012 16:09 UTC (Thu) by djm1021 (subscriber, #31130) [Link]
What you suggest is certainly possible (but anything is possible in software :-), however it would take fairly massive changes to the swap subsystem and/or userland-visible changes. Currently, the swap cache is an in-RAM mapping of some subset of pages on the swap disk and this is important for maintaining coherency between the (one or more user or kernel) threads accessing those pages and the I/O reading/writing the swap disk. If some pageframe-size slots on the swapdisk contain one or more compressed pages, this becomes a lot more complex.
In the end, such changes might be necessary or be the best alternative, but we are trying less invasive approaches first.
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