Re: [RFC] block: Change the granularity of io ticks from ms to ns

[Paul Menzel]

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@xxxxxxxxxx>
> Signed-off-by: Gulam Mohamed <gulam.mohamed@xxxxxxxxxx>

(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