Re: [RFC PATCH 2/2] mm: introduce budgt control in readahead

[Zhaoyang Huang <huangzhaoyang@gmail.com>]

On Wed, May 15, 2024 at 3:40 PM Tejun Heo <tj@kernel.org> wrote:
>
> Hello,
>
> On Wed, May 15, 2024 at 09:23:50AM +0800, zhaoyang.huang wrote:
> > +static unsigned long get_next_ra_size(struct readahead_control *ractl,
> >                                     unsigned long max)
> >  {
> > +     unsigned long cur = ractl->ra->size;
> > +     struct inode *inode = ractl->mapping->host;
> > +     unsigned long budgt = inode->i_sb->s_bdev ?
> > +                     blk_throttle_budgt(inode->i_sb->s_bdev) : 0;
>
> Technical correctness aside, I'm not convinced it's generally a good idea to
> bubble up one specific IO control mechanism's detail all the way upto RA
> layer. Besides what's the gain here? For continuous IO stream, whether some
> RA bios are oversized or not shouldn't matter, no? Doesn't this just affect
> the accuracy of the last RA IO of a finite read stream?
Thanks for feedback. If I understand right, the oversized RA bios of a
finite read will fail by being queued to tg's queue which should be
deemed as introducing a drop of IOPS.

submit_bio
    blk_throtl_bio
        if(!tg_may_dispatch) //failed, queue the bio to tg's queue

What we get here is a more precise BW of the throttled blkcg like
below, from which we can find the result of 'after' could exactly meet
the configured bps value and a little bit enhancement since there are
no hung(oversized) bios any more.

blkio.throttle.read_bps_device = 20MB/s
fio ... -numjobs=8 ...

    before : IOPS=37.9k, BW=148MiB/s (155MB/s)(11.6GiB/80333msec)
    after  : IOPS=39.0k, BW=153MiB/s (160MB/s)(15.6GiB/104914msec)

    before : clat (usec): min=4, max=1056.6k, avg=197.23, stdev=10080.69
    after  : clat (usec): min=4, max=193481, avg=188.83, stdev=4651.29

    before : lat (usec): min=5, max=1056.6k, avg=200.48, stdev=10080.76
    after  : lat (usec): min=5, max=193483, avg=192.68, stdev=4651.87

blkio.throttle.read_bps_device = 30MB/s
fio ... -numjobs=8 ...

    before : IOPS=57.2k, BW=224MiB/s (234MB/s)(15.6GiB/71561msec)
    after  : IOPS=58.5k, BW=229MiB/s (240MB/s)(15.6GiB/69996msec)

    before : clat (usec): min=4, max=1105.5k, avg=126.20, stdev=6419.22
    after  : clat (usec): min=4, max=183956, avg=120.60, stdev=2957.28

    before : lat (usec): min=5, max=1105.5k, avg=129.45, stdev=6419.29
    after  : lat (usec): min=5, max=183958, avg=124.40, stdev=2958.18

>
> Thanks. blk_throttle_budgt

>
> --
> tejun