On Fri, Mar 10, 2023 at 11:06:37AM +0800, Huang, Ying wrote: > > Unfortunately it's a little bit more. 24 is the extra overhead. > > > > Today we have an xarray entry for each swapped out page, that either > > has the swapcache pointer or the shadow entry. > > > > With this implementation, we have an xarray entry for each swapped out > > page, that has a pointer to the swap_desc. > > > > Ignoring the overhead of the xarray itself, we have (8 + 24) / (8 + 1) = 3.5556. > > OK. I see. We can only hold 8 bytes for each xarray entry. To save > memory usage, we can allocate multiple swap_desc (e.g., 16) for each > xarray entry. Then the memory usage of xarray becomes 1/N. The xarray look up key is the swap offset from the swap entry. If you put more than one swap_desc under the one xarray entry. It will mean all those different swap_descs will share a swap offset. > >> > Yeah, I initially thought we would only need the swp_entry_t -> > >> > swap_desc reverse mapping for readahead, and that we can only store > >> > that for spinning disks, but I was wrong. We need for other things as > >> > well today: swapoff, when trying to find an empty swap slot and we > >> > start trying to free swap slots used only by the swapcache. However, I > >> > think both of these cases can be fixed (I can share more details if > >> > you want). If everything goes well we should only need to maintain the > >> > reverse mapping (extra overhead above 24 bytes) for swap files on > >> > spinning disks for readahead. > >> > > >> >> > >> >> Looking forward to your discussion. > > Per my understanding, the indirection is to make it easy to move > (swapped) pages among swap devices based on hot/cold. This is similar > as the target of memory tiering. It appears that we can extend the > memory tiering (mm/memory-tiers.c) framework to cover swap devices too? > Is it possible for zswap to be faster than some slow memory media? I just took a look at the mm/memory-tier.c. It does not look like it can cover block device swap without major overhauling. Chris
