Re: [RFCv2 3/3] iomap: Support subpage size dirty tracking to improve write performance

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 30, 2023 at 09:44:13PM +0530, Ritesh Harjani (IBM) wrote:
> On a 64k pagesize platforms (specially Power and/or aarch64) with 4k
> filesystem blocksize, this patch should improve the performance by doing
> only the subpage dirty data write.
> 
> This should also reduce the write amplification since we can now track
> subpage dirty status within state bitmaps. Earlier we had to
> write the entire 64k page even if only a part of it (e.g. 4k) was
> updated.
> 
> Performance testing of below fio workload reveals ~16x performance
> improvement on nvme with XFS (4k blocksize) on Power (64K pagesize)
> FIO reported write bw scores improved from around ~28 MBps to ~452 MBps.
> 
> <test_randwrite.fio>
> [global]
> 	ioengine=psync
> 	rw=randwrite
> 	overwrite=1
> 	pre_read=1
> 	direct=0
> 	bs=4k
> 	size=1G
> 	dir=./
> 	numjobs=8
> 	fdatasync=1
> 	runtime=60
> 	iodepth=64
> 	group_reporting=1
> 
> [fio-run]

You really need to include this sentence from the cover letter in this
patch:

2. Also our internal performance team reported that this patch improves there
   database workload performance by around ~83% (with XFS on Power)

because that's far more meaningful than "Look, I cooked up an artificial
workload where this makes a difference".