On Wed, Jun 12, 2024 at 5:24 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > On Tue, Jun 11, 2024 at 12:23:41PM GMT, Barry Song wrote: > > On Tue, Jun 11, 2024 at 8:43 AM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > > > > > On Thu, Mar 14, 2024 at 08:56:17PM GMT, Chuanhua Han wrote: > > > [...] > > > > > > > > > > So in the common case, swap-in will pull in the same size of folio as was > > > > > swapped-out. Is that definitely the right policy for all folio sizes? Certainly > > > > > it makes sense for "small" large folios (e.g. up to 64K IMHO). But I'm not sure > > > > > it makes sense for 2M THP; As the size increases the chances of actually needing > > > > > all of the folio reduces so chances are we are wasting IO. There are similar > > > > > arguments for CoW, where we currently copy 1 page per fault - it probably makes > > > > > sense to copy the whole folio up to a certain size. > > > > For 2M THP, IO overhead may not necessarily be large? :) > > > > 1.If 2M THP are continuously stored in the swap device, the IO > > > > overhead may not be very large (such as submitting bio with one > > > > bio_vec at a time). > > > > 2.If the process really needs this 2M data, one page-fault may perform > > > > much better than multiple. > > > > 3.For swap devices like zram,using 2M THP might also improve > > > > decompression efficiency. > > > > > > > > > > Sorry for late response, do we have any performance data backing the > > > above claims particularly for zswap/swap-on-zram cases? > > > > no need to say sorry. You are always welcome to give comments. > > > > this, combining with zram modification, not only improves compression > > ratio but also reduces CPU time significantly. you may find some data > > here[1]. > > > > granularity orig_data_size compr_data_size time(us) > > 4KiB-zstd 1048576000 246876055 50259962 > > 64KiB-zstd 1048576000 199763892 18330605 > > > > On mobile devices, We tested the performance of swapin by running > > 100 iterations of swapping in 100MB of data ,and the results were > > as follows.the swapin speed increased by about 45%. > > > > time consumption of swapin(ms) > > lz4 4k 45274 > > lz4 64k 22942 > > > > zstdn 4k 85035 > > zstdn 64k 46558 > > Thanks for the response. Above numbers are actually very fascinating and > counter intuitive (at least to me). Do you also have numbers for 2MiB > THP? I am assuming 64k is the right balance between too small or too > large. Did you experiment on server machines as well? I don’t possess data on 2MiB, and regrettably, I lack a server machine for testing. However, I believe that this type of higher compression ratio and lower CPU consumption generally holds true for generic anonymous memory. 64KB is a right balance. But nothing can stop THP from using 64KB to swapin, compression and decompression. as you can see from the zram/zsmalloc series, we actually have a configuration CONFIG_ZSMALLOC_MULTI_PAGES_ORDER The default value is 4. That means a 2MB THP can be compressed/decompressed as 32 * 64KB. If we use 64KB as the swapin granularity, we still have the balance and all the benefits if 2MB is a too large swap-in granularity which might cause memory waste. > > > > > [1] https://lore.kernel.org/linux-mm/20240327214816.31191-1-21cnbao@xxxxxxxxx/ > > Thanks Barry
