Re: [RFC 11/11] ext4: Add allocation criteria 1.5 (CR1_5)

Jan Kara
2023-03-09T15:10:34Z

On Fri 27-01-23 18:07:38, Ojaswin Mujoo wrote:
> CR1_5 aims to optimize allocations which can't be satisfied in CR1. The
> fact that we couldn't find a group in CR1 suggests that it would be
> difficult to find a continuous extent to compleltely satisfy our
> allocations. So before falling to the slower CR2, in CR1.5 we
> proactively trim the the preallocations so we can find a group with
> (free / fragments) big enough. This speeds up our allocation at the
> cost of slightly reduced preallocation.
>
> The patch also adds a new sysfs tunable:
>
> * /sys/fs/ext4/<partition>/mb_cr1_5_max_trim_order
>
> This controls how much CR1.5 can trim a request before falling to CR2.
> For example, for a request of order 7 and max trim order 2, CR1.5 can
> trim this upto order 5.
>
> Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>

The idea looks good. Couple of questions below...

> +/*
> + * We couldn't find a group in CR1 so try to find the highest free fragment
> + * order we have and proactively trim the goal request length to that order to
> + * find a suitable group faster.
> + *
> + * This optimizes allocation speed at the cost of slightly reduced
> + * preallocations. However, we make sure that we don't trim the request too
> + * much and fall to CR2 in that case.
> + */
> +static void ext4_mb_choose_next_group_cr1_5(struct ext4_allocation_context *ac,
> + enum criteria *new_cr, ext4_group_t *group, ext4_group_t ngroups)
> +{
> + struct ext4_sb_info *sbi = EXT4_SB(ac->ac_sb);
> + struct ext4_group_info *grp = NULL;
> + int i, order, min_order;
> +
> + if (unlikely(ac->ac_flags & EXT4_MB_CR1_5_OPTIMIZED)) {
> + if (sbi->s_mb_stats)
> + atomic_inc(&sbi->s_bal_cr1_5_bad_suggestions);
> + }
> +
> + /*
> + * mb_avg_fragment_size_order() returns order in a way that makes
> + * retrieving back the length using (1 << order) inaccurate. Hence, use
> + * fls() instead since we need to know the actual length while modifying
> + * goal length.
> + */
> + order = fls(ac->ac_g_ex.fe_len);
> + min_order = order - sbi->s_mb_cr1_5_max_trim_order;

Given we still require the allocation contains at least originally
requested blocks, is it ever the case that goal size would be 8 times
larger than original alloc size? Otherwise the
sbi->s_mb_cr1_5_max_trim_order logic seems a bit pointless...

> + if (min_order < 0)
> + min_order = 0;

Perhaps add:

if (1 << min_order < ac->ac_o_ex.fe_len)
min_order = fls(ac->ac_o_ex.fe_len) + 1;

and then you can drop the condition from the loop below...

> +
> + for (i = order; i >= min_order; i--) {
> + if (ac->ac_o_ex.fe_len <= (1 << i)) {
> + /*
> + * Scale down goal len to make sure we find something
> + * in the free fragments list. Basically, reduce
> + * preallocations.
> + */
> + ac->ac_g_ex.fe_len = 1 << i;

When scaling down the size with sbi->s_stripe > 1, it would be better to
choose multiple of sbi->s_stripe and not power of two. But our stripe
support is fairly weak anyway (e.g. initial goal size does not reflect it
at all AFAICT) so probably we don't care here either.

> + } else {
> + break;
> + }
> +
> + grp = ext4_mb_find_good_group_avg_frag_lists(ac,
> + mb_avg_fragment_size_order(ac->ac_sb,
> + ac->ac_g_ex.fe_len));
> + if (grp)
> + break;
> + }
> +
> + if (grp) {
> + *group = grp->bb_group;
> + ac->ac_flags |= EXT4_MB_CR1_5_OPTIMIZED;
> + } else {
> + /* Reset goal length to original goal length before falling into CR2 */
> + ac->ac_g_ex.fe_len = ac->ac_orig_goal_len;
> *new_cr = CR2;
> }
> }

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR