Re: [RFC PATCH 0/1] Large folios in block buffered IO path

Bharata B Rao
2024-12-02T09:37:36Z

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On 28-Nov-24 10:01 AM, Mateusz Guzik wrote:

> WIlly mentioned the folio wait queue hash table could be grown, you
> can find it in mm/filemap.c:
> 1062 #define PAGE_WAIT_TABLE_BITS 8
> 1063 #define PAGE_WAIT_TABLE_SIZE (1 << PAGE_WAIT_TABLE_BITS)
> 1064 static wait_queue_head_t folio_wait_table[PAGE_WAIT_TABLE_SIZE]
> __cacheline_aligned;
> 1065
> 1066 static wait_queue_head_t *folio_waitqueue(struct folio *folio)
> 1067 {
> 1068 │ return &folio_wait_table[hash_ptr(folio, PAGE_WAIT_TABLE_BITS)];
> 1069 }
>
> Can you collect off cpu time? offcputime-bpfcc -K > /tmp/out

Flamegraph for "perf record --off-cpu -F 99 -a -g --all-kernel
--kernel-callchains -- sleep 120" is attached.

Off-cpu samples were collected for 120s at around 45th minute run of the
FIO benchmark that actually runs for 1hr. This run was with kernel that
had your inode_lock fix but no changes to PAGE_WAIT_TABLE_BITS.

Hopefully this captures the representative sample of the scalability
issue with folio lock.

Regards,
Bharata.

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