On Wed 30-04-25 15:59:17, Ryan Roberts wrote: > Previously the folio order of the previous readahead request was > inferred from the folio who's readahead marker was hit. But due to the > way we have to round to non-natural boundaries sometimes, this first > folio in the readahead block is often smaller than the preferred order > for that request. This means that for cases where the initial sync > readahead is poorly aligned, the folio order will ramp up much more > slowly. > > So instead, let's store the order in struct file_ra_state so we are not > affected by any required alignment. We previously made enough room in > the struct for a 16 order field. This should be plenty big enough since > we are limited to MAX_PAGECACHE_ORDER anyway, which is certainly never > larger than ~20. > > Since we now pass order in struct file_ra_state, page_cache_ra_order() > no longer needs it's new_order parameter, so let's remove that. > > Worked example: > > Here we are touching pages 17-256 sequentially just as we did in the > previous commit, but now that we are remembering the preferred order > explicitly, we no longer have the slow ramp up problem. Note > specifically that we no longer have 2 rounds (2x ~128K) of order-2 > folios: > > TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER RA > ----- ---------- ---------- ---------- ------- ------- ----- ----- -- > HOLE 0x00000000 0x00001000 4096 0 1 1 > FOLIO 0x00001000 0x00002000 4096 1 2 1 0 > FOLIO 0x00002000 0x00003000 4096 2 3 1 0 > FOLIO 0x00003000 0x00004000 4096 3 4 1 0 > FOLIO 0x00004000 0x00005000 4096 4 5 1 0 > FOLIO 0x00005000 0x00006000 4096 5 6 1 0 > FOLIO 0x00006000 0x00007000 4096 6 7 1 0 > FOLIO 0x00007000 0x00008000 4096 7 8 1 0 > FOLIO 0x00008000 0x00009000 4096 8 9 1 0 > FOLIO 0x00009000 0x0000a000 4096 9 10 1 0 > FOLIO 0x0000a000 0x0000b000 4096 10 11 1 0 > FOLIO 0x0000b000 0x0000c000 4096 11 12 1 0 > FOLIO 0x0000c000 0x0000d000 4096 12 13 1 0 > FOLIO 0x0000d000 0x0000e000 4096 13 14 1 0 > FOLIO 0x0000e000 0x0000f000 4096 14 15 1 0 > FOLIO 0x0000f000 0x00010000 4096 15 16 1 0 > FOLIO 0x00010000 0x00011000 4096 16 17 1 0 > FOLIO 0x00011000 0x00012000 4096 17 18 1 0 > FOLIO 0x00012000 0x00013000 4096 18 19 1 0 > FOLIO 0x00013000 0x00014000 4096 19 20 1 0 > FOLIO 0x00014000 0x00015000 4096 20 21 1 0 > FOLIO 0x00015000 0x00016000 4096 21 22 1 0 > FOLIO 0x00016000 0x00017000 4096 22 23 1 0 > FOLIO 0x00017000 0x00018000 4096 23 24 1 0 > FOLIO 0x00018000 0x00019000 4096 24 25 1 0 > FOLIO 0x00019000 0x0001a000 4096 25 26 1 0 > FOLIO 0x0001a000 0x0001b000 4096 26 27 1 0 > FOLIO 0x0001b000 0x0001c000 4096 27 28 1 0 > FOLIO 0x0001c000 0x0001d000 4096 28 29 1 0 > FOLIO 0x0001d000 0x0001e000 4096 29 30 1 0 > FOLIO 0x0001e000 0x0001f000 4096 30 31 1 0 > FOLIO 0x0001f000 0x00020000 4096 31 32 1 0 > FOLIO 0x00020000 0x00021000 4096 32 33 1 0 > FOLIO 0x00021000 0x00022000 4096 33 34 1 0 > FOLIO 0x00022000 0x00024000 8192 34 36 2 1 > FOLIO 0x00024000 0x00028000 16384 36 40 4 2 > FOLIO 0x00028000 0x0002c000 16384 40 44 4 2 > FOLIO 0x0002c000 0x00030000 16384 44 48 4 2 > FOLIO 0x00030000 0x00034000 16384 48 52 4 2 > FOLIO 0x00034000 0x00038000 16384 52 56 4 2 > FOLIO 0x00038000 0x0003c000 16384 56 60 4 2 > FOLIO 0x0003c000 0x00040000 16384 60 64 4 2 > FOLIO 0x00040000 0x00050000 65536 64 80 16 4 > FOLIO 0x00050000 0x00060000 65536 80 96 16 4 > FOLIO 0x00060000 0x00080000 131072 96 128 32 5 > FOLIO 0x00080000 0x000a0000 131072 128 160 32 5 > FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5 > FOLIO 0x000c0000 0x000e0000 131072 192 224 32 5 > FOLIO 0x000e0000 0x00100000 131072 224 256 32 5 > FOLIO 0x00100000 0x00120000 131072 256 288 32 5 > FOLIO 0x00120000 0x00140000 131072 288 320 32 5 Y > HOLE 0x00140000 0x00800000 7077888 320 2048 1728 > > Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx> ... > @@ -469,6 +469,7 @@ void page_cache_ra_order(struct readahead_control *ractl, > int err = 0; > gfp_t gfp = readahead_gfp_mask(mapping); > unsigned int min_ra_size = max(4, mapping_min_folio_nrpages(mapping)); > + unsigned int new_order = ra->order; > > /* > * Fallback when size < min_nrpages as each folio should be > @@ -483,6 +484,8 @@ void page_cache_ra_order(struct readahead_control *ractl, > new_order = min_t(unsigned int, new_order, ilog2(ra->size)); > new_order = max(new_order, min_order); > > + ra->order = new_order; > + > /* See comment in page_cache_ra_unbounded() */ > nofs = memalloc_nofs_save(); > filemap_invalidate_lock_shared(mapping); > @@ -525,6 +528,7 @@ void page_cache_ra_order(struct readahead_control *ractl, > * ->readahead() may have updated readahead window size so we have to > * check there's still something to read. > */ > + ra->order = 0; Hum, so you reset desired folio order if readahead hit some pre-existing pages in the page cache. Is this really desirable? Why not leave the desired order as it was for the next request? > if (ra->size > index - start) > do_page_cache_ra(ractl, ra->size - (index - start), > ra->async_size); Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
