Re: [LKP] [btrfs] 302167c50b: fio.write_bw_MBps -12.4% regression

[Josef Bacik]

On Fri, May 24, 2019 at 03:46:17PM +0800, Huang, Ying wrote:
> "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
> 
> > "Huang, Ying" <ying.huang@xxxxxxxxx> writes:
> >
> >> Hi, Josef,
> >>
> >> kernel test robot <rong.a.chen@xxxxxxxxx> writes:
> >>
> >>> Greeting,
> >>>
> >>> FYI, we noticed a -12.4% regression of fio.write_bw_MBps due to commit:
> >>>
> >>>
> >>> commit: 302167c50b32e7fccc98994a91d40ddbbab04e52 ("btrfs: don't end
> >>> the transaction for delayed refs in throttle")
> >>> https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git pending-fixes
> >>>
> >>> in testcase: fio-basic
> >>> on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
> >>> with following parameters:
> >>>
> >>> 	runtime: 300s
> >>> 	nr_task: 8t
> >>> 	disk: 1SSD
> >>> 	fs: btrfs
> >>> 	rw: randwrite
> >>> 	bs: 4k
> >>> 	ioengine: sync
> >>> 	test_size: 400g
> >>> 	cpufreq_governor: performance
> >>> 	ucode: 0xb00002e
> >>>
> >>> test-description: Fio is a tool that will spawn a number of threads
> >>> or processes doing a particular type of I/O action as specified by
> >>> the user.
> >>> test-url: https://github.com/axboe/fio
> >>>
> >>>
> >>
> >> Do you have time to take a look at this regression?
> >
> > Ping
> 
> Ping again.
> 

This happens because now we rely more on on-demand flushing than the catchup
flushing that happened before.  This is just one case where it's slightly worse,
overall this change provides better latencies, and even in this result it
provided better completion latencies because we're not randomly flushing at the
end of a transaction.  It does appear to be costing writes in that they will
spend more time flushing than before, so you get slightly lower throughput on
pure small write workloads.  I can't actually see the slowdown locally.

This patch is here to stay, it just shows we need to continue to refine the
flushing code to be less spikey/painful.  Thanks,

Josef