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

David Hildenbrand
2025-05-05T10:27:58Z

On 05.05.25 12:09, Jan Kara wrote:
> 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.

Ah, thanks for that information! :)

--
Cheers,

David / dhildenb