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Message-ID: <b28ab260-7a5a-4801-4d33-80c66ae25f89@molgen.mpg.de>
Date: Tue, 6 Dec 2022 20:31:27 +0100
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Gulam Mohamed <gulam.mohamed@...cle.com>
Cc: linux-block@...r.kernel.org, axboe@...nel.dk,
philipp.reisner@...bit.com, lars.ellenberg@...bit.com,
christoph.boehmwalder@...bit.com, minchan@...nel.org,
ngupta@...are.org, senozhatsky@...omium.org, colyli@...e.de,
kent.overstreet@...il.com, agk@...hat.com, snitzer@...nel.org,
dm-devel@...hat.com, song@...nel.org, dan.j.williams@...el.com,
vishal.l.verma@...el.com, dave.jiang@...el.com,
ira.weiny@...el.com, junxiao.bi@...cle.com,
martin.petersen@...cle.com, kch@...dia.com,
drbd-dev@...ts.linbit.com, linux-kernel@...r.kernel.org,
linux-bcache@...r.kernel.org, linux-raid@...r.kernel.org,
nvdimm@...ts.linux.dev, konrad.wilk@...cle.com
Subject: Re: [RFC] block: Change the granularity of io ticks from ms to ns
Dear Gulam,
Thank you for the patch.
Am 06.12.22 um 19:15 schrieb Gulam Mohamed:
> Use ktime to change the granularity of IO accounting in block layer from
> milli-seconds to nano-seconds to get the proper latency values for the
> devices whose latency is in micro-seconds. After changing the granularity
> to nano-seconds the iostat command, which was showing incorrect values for
> %util, is now showing correct values.
>
> We did not work on the patch to drop the logic for
> STAT_PRECISE_TIMESTAMPS yet. Will do it if this patch is ok.
>
> The iostat command was run after starting the fio with following command
> on an NVME disk. For the same fio command, the iostat %util was showing
> ~100% for the disks whose latencies are in the range of microseconds.
> With the kernel changes (granularity to nano-seconds), the %util was
> showing correct values. Following are the details of the test and their
> output:
>
> fio command
> -----------
> [global]
> bs=128K
> iodepth=1
> direct=1
> ioengine=libaio
> group_reporting
> time_based
> runtime=90
> thinktime=1ms
> numjobs=1
> name=raw-write
> rw=randrw
> ignore_error=EIO:EIO
> [job1]
> filename=/dev/nvme0n1
>
> Correct values after kernel changes:
> ====================================
> iostat output
> -------------
> iostat -d /dev/nvme0n1 -x 1
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.08 0.05 0.06 128.00 128.00 0.07 6.50
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.08 0.06 0.06 128.00 128.00 0.07 6.30
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.06 0.05 0.06 128.00 128.00 0.06 5.70
>
> From fio
> --------
> Read Latency: clat (usec): min=32, max=2335, avg=79.54, stdev=29.95
> Write Latency: clat (usec): min=38, max=130, avg=57.76, stdev= 3.25
>
> Values before kernel changes
> ============================
> iostat output
> -------------
>
> iostat -d /dev/nvme0n1 -x 1
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.08 0.06 0.06 128.00 128.00 1.07 97.70
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.08 0.06 0.06 128.00 128.00 1.08 98.80
>
> Device r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
> nvme0n1 0.08 0.05 0.06 128.00 128.00 1.06 97.20
>
> From fio
> --------
> Read Latency: clat (usec): min=33, max=468, avg=79.56, stdev=28.04
> Write Latency: clat (usec): min=9, max=139, avg=57.10, stdev= 3.79
I’d order the numbers before the change first, but it’s not important.
> Signed-off-by: Junxiao Bi <junxiao.bi@...cle.com>
> Signed-off-by: Gulam Mohamed <gulam.mohamed@...cle.com>
(If Junxiao is the patch author, that name should be documented as the
patch author.)
> ---
> block/blk-core.c | 26 +++++++++++++-------------
> block/blk-mq.c | 4 ++--
> block/blk.h | 2 +-
> block/genhd.c | 8 ++++----
> drivers/block/drbd/drbd_debugfs.c | 4 ++--
> drivers/block/drbd/drbd_int.h | 2 +-
> drivers/block/zram/zram_drv.c | 4 ++--
> drivers/md/bcache/request.c | 10 +++++-----
> drivers/md/dm-core.h | 2 +-
> drivers/md/dm.c | 8 ++++----
> drivers/md/md.h | 2 +-
> drivers/md/raid1.h | 2 +-
> drivers/md/raid10.h | 2 +-
> drivers/md/raid5.c | 2 +-
> drivers/nvdimm/btt.c | 2 +-
> drivers/nvdimm/pmem.c | 2 +-
> include/linux/blk_types.h | 2 +-
> include/linux/blkdev.h | 14 +++++++-------
> include/linux/part_stat.h | 2 +-
> 19 files changed, 50 insertions(+), 50 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 5487912befe8..069c29dad3e7 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -912,12 +912,12 @@ int iocb_bio_iopoll(struct kiocb *kiocb, struct io_comp_batch *iob,
> }
> EXPORT_SYMBOL_GPL(iocb_bio_iopoll);
>
> -void update_io_ticks(struct block_device *part, unsigned long now, bool end)
> +void update_io_ticks(struct block_device *part, u64 now, bool end)
> {
> - unsigned long stamp;
> + u64 stamp;
Did you check the code size difference with `scripts/bloat-o-meter`?
Instead of the fixed-size type, I’d use `unsigned long long` to ensure
the minimum size is 64-bit.
> again:
> stamp = READ_ONCE(part->bd_stamp);
> - if (unlikely(time_after(now, stamp))) {
> + if (unlikely(time_after64(now, stamp))) {
No idea, if it’s possible with `time_after64()` though, as that uses
`__u64`.
[…]
Kind regards,
Paul
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