Re: [RFC PATCH v4 1/5] mm/readahead: Honour new_order in page_cache_ra_order()

Jan Kara
2025-05-05T10:15:16Z

On Mon 05-05-25 11:51:43, David Hildenbrand wrote:
> On 30.04.25 16:59, Ryan Roberts wrote:
> > page_cache_ra_order() takes a parameter called new_order, which is
> > intended to express the preferred order of the folios that will be
> > allocated for the readahead operation. Most callers indeed call this
> > with their preferred new order. But page_cache_async_ra() calls it with
> > the preferred order of the previous readahead request (actually the
> > order of the folio that had the readahead marker, which may be smaller
> > when alignment comes into play).
> >
> > And despite the parameter name, page_cache_ra_order() always treats it
> > at the old order, adding 2 to it on entry. As a result, a cold readahead
> > always starts with order-2 folios.
> >
> > Let's fix this behaviour by always passing in the *new* order.
> >
> > Worked example:
> >
> > Prior to the change, mmaping an 8MB file and touching each page
> > sequentially, resulted in the following, where we start with order-2
> > folios for the first 128K then ramp up to order-4 for the next 128K,
> > then get clamped to order-5 for the rest of the file because pa_pages is
> > limited to 128K:
> >
> > TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER
> > ----- ---------- ---------- --------- ------- ------- ----- -----
> > FOLIO 0x00000000 0x00004000 16384 0 4 4 2
> > FOLIO 0x00004000 0x00008000 16384 4 8 4 2
> > FOLIO 0x00008000 0x0000c000 16384 8 12 4 2
> > FOLIO 0x0000c000 0x00010000 16384 12 16 4 2
> > FOLIO 0x00010000 0x00014000 16384 16 20 4 2
> > FOLIO 0x00014000 0x00018000 16384 20 24 4 2
> > FOLIO 0x00018000 0x0001c000 16384 24 28 4 2
> > FOLIO 0x0001c000 0x00020000 16384 28 32 4 2
> > FOLIO 0x00020000 0x00030000 65536 32 48 16 4
> > FOLIO 0x00030000 0x00040000 65536 48 64 16 4
> > FOLIO 0x00040000 0x00060000 131072 64 96 32 5
> > FOLIO 0x00060000 0x00080000 131072 96 128 32 5
> > FOLIO 0x00080000 0x000a0000 131072 128 160 32 5
> > FOLIO 0x000a0000 0x000c0000 131072 160 192 32 5
>
> Interesting, I would have thought we'd ramp up earlier.
>
> > ...
> >
> > After the change, the same operation results in the first 128K being
> > order-0, then we start ramping up to order-2, -4, and finally get
> > clamped at order-5:
> >
> > TYPE STARTOFFS ENDOFFS SIZE STARTPG ENDPG NRPG ORDER
> > ----- ---------- ---------- --------- ------- ------- ----- -----
> > FOLIO 0x00000000 0x00001000 4096 0 1 1 0
> > 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 0x00024000 16384 32 36 4 2
> > 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
>
> Similar here, do you know why we don't ramp up earlier. Allocating that many
> order-0 + order-2 pages looks a bit suboptimal to me for a sequential read.

Note that this is reading through mmap using the mmap readahead code. If
you use standard read(2), the readahead window starts small as well and
ramps us along with the desired order so we don't allocate that many small
order pages in that case.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR