Re: sequential 1MB mmap read ends in 1 page sync read-ahead

[Bernd Schubert <bernd.schubert@fastmail.fm>]

On 4/14/23 17:05, Bernd Schubert wrote:
> 
> 
> On 4/14/23 06:07, Matthew Wilcox wrote:
>> On Thu, Apr 13, 2023 at 11:33:09PM +0200, Bernd Schubert wrote:
>>> Sorry, forgot to add Andrew and linux-mm into CC.
>>>
>>> On 4/13/23 23:27, Bernd Schubert wrote:
>>>> Hello,
>>>>
>>>> I found a weird mmap read behavior while benchmarking the fuse-uring
>>>> patches.
>>>> I did not verify yet, but it does not look fuse specific.
>>>> Basically, I started to check because fio results were much lower
>>>> than expected (better with the new code, though)
>>>>
>>>> fio cmd line:
>>>> fio --size=1G --numjobs=1 --ioengine=mmap --output-format=normal,terse
>>>> --directory=/scratch/dest/ --rw=read multi-file.fio
>>>>
>>>>
>>>> bernd@squeeze1 test2>cat multi-file.fio
>>>> [global]
>>>> group_reporting
>>>> bs=1M
>>>> runtime=300
>>>>
>>>> [test]
>>>>
>>>> This sequential fio sets POSIX_MADV_SEQUENTIAL and then does memcpy
>>>> beginning at offset 0 in 1MB steps (verified with additional
>>>> logging in fios engines/mmap.c).
>>>>
>>>> And additional log in fuse_readahead() gives
>>>>
>>>> [ 1396.215084] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64
>>>> index=0
>>>> [ 1396.237466] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64
>>>> index=255
>>>> [ 1396.263175] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1
>>>> index=254
>>>> [ 1396.282055] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1
>>>> index=253
>>>> ... <count is always 1 page>
>>>> [ 1496.353745] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1
>>>> index=64
>>>> [ 1496.381105] fuse: 000000003fdec504 inode=00000000be0f29d3 count=64
>>>> index=511
>>>> [ 1496.397487] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1
>>>> index=510
>>>> [ 1496.416385] fuse: 000000003fdec504 inode=00000000be0f29d3 count=1
>>>> index=509
>>>> ... <count is always 1 page>
>>>>
>>>> Logging in do_sync_mmap_readahead()
>>>>
>>>> [ 1493.130764] do_sync_mmap_readahead:3015 ino=132 index=0 count=0
>>>> ras_start=0 ras_size=0 ras_async=0 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>> [ 1493.147173] do_sync_mmap_readahead:3015 ino=132 index=255 count=0
>>>> ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>> [ 1493.165952] do_sync_mmap_readahead:3015 ino=132 index=254 count=0
>>>> ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>> [ 1493.185566] do_sync_mmap_readahead:3015 ino=132 index=253 count=0
>>>> ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>> ...
>>>> [ 1496.341890] do_sync_mmap_readahead:3015 ino=132 index=64 count=0
>>>> ras_start=0 ras_size=64 ras_async=32 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>> [ 1496.361385] do_sync_mmap_readahead:3015 ino=132 index=511 count=0
>>>> ras_start=96 ras_size=64 ras_async=64 ras_ra_pages=64 ras_mmap_miss=0
>>>> ras_prev_pos=-1
>>>>
>>>>
>>>> So we can see from fuse that it starts to read at page index 0, wants
>>>> 64 pages (which is actually the double of bdi read_ahead_kb), then
>>>> skips index 64...254) and immediately goes to index 255. For the mmaped
>>>> memcpy pages are missing and then it goes back in 1 page steps to get
>>>> these.
>>>>
>>>> A workaround here is to set read_ahead_kb in the bdi to a larger
>>>> value, another workaround might be (untested) to increase the 
>>>> read-ahead
>>>> window. Either of these two seem to be workarounds for the index order
>>>> above.
>>>>
>>>> I understand that read-ahead gets limited by the bdi value (although
>>>> exceeded above), but why does it go back in 1 page steps? My 
>>>> expectation
>>>> would have been
>>>>
>>>> index=0  count=32 (128kb read-head)
>>>> index=32 count=32
>>>> index=64 count=32
>>
>> What I see with XFS is:
>>
>>               fio-27518   [005] .....   276.565025: 
>> mm_filemap_add_to_page_cache: dev 8:32 ino 44 pfn=0x23a8c ofs=0 order=2

...

>> mm_filemap_add_to_page_cache: dev 8:32 ino 44 pfn=0x15e40 ofs=786432 
>> order=6
>>
>> ... it then gets "stuck" at order-6, which is expected for a 256kB
>> readahead window.
>>
>> This is produced by:
>>
>> echo 1 
>> >/sys/kernel/tracing/events/filemap/mm_filemap_add_to_page_cache/enable
>> fio --size=1G --numjobs=1 --ioengine=mmap --output-format=normal,terse 
>> --directory=/mnt/scratch/ --rw=read multi-file.fio
>> echo 0 
>> >/sys/kernel/tracing/events/filemap/mm_filemap_add_to_page_cache/enable
>> less /sys/kernel/tracing/trace
>>
> 
> Thanks for looking at it Matthew!
> 
> I see the same as on fuse on xfs - same output as I initially
> posted (except the fuse logging, of course).
> 
> With tracing and reduced file size to 2M
> 

...

>               fio-3459    [011] ..... 65055.436534: 
> mm_filemap_add_to_page_cache: dev 252:16 ino 84 pfn=0x12cfb5 ofs=1052672 
> order=0
> 
> And then it is stuck at order=0
> 

When I reduce bs to 4K I get similar results as you

fio --size=10M --numjobs=1 --ioengine=mmap --output-format=normal,terse --directory=/scratch/source/ --rw=read multi-file.fio --bs=4K --group_reporting

(so bs set to 4K)

and a basically empty job file

bernd@squeeze1 test2>cat multi-file.fio
[global]

[test]



Up to bs=512K it works fine, 1M (and for what it matters
already 768K) introduce the order=0 issue.


Thanks,
Bernd