Re: [PATCH v2 11/12] ext4: Add allocation criteria 1.5 (CR1_5)

[Jan Kara]

On Tue 30-05-23 18:03:49, 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.
> 
> Suggested-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
> Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> 
> ext4 squash

Why is this here?

> +/*
> + * 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;
> +	unsigned long num_stripe_clusters = 0;
> +
> +	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;
> +	if (min_order < 0)
> +		min_order = 0;
> +
> +	if (1 << min_order < ac->ac_o_ex.fe_len)
> +		min_order = fls(ac->ac_o_ex.fe_len) + 1;
> +
> +	if (sbi->s_stripe > 0) {
> +		/*
> +		 * We are assuming that stripe size is always a multiple of
> +		 * cluster ratio otherwise __ext4_fill_super exists early.
> +		 */
> +		num_stripe_clusters = EXT4_NUM_B2C(sbi, sbi->s_stripe);
> +		if (1 << min_order < num_stripe_clusters)
> +			min_order = fls(num_stripe_clusters);
> +	}
> +
> +	for (i = order; i >= min_order; i--) {
> +		int frag_order;
> +		/*
> +		 * 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;

I smell some off-by-one issues here. Look fls(1) == 1 so (1 << fls(n)) > n.
Hence this loop will actually *grow* the goal allocation length. Also I'm
not sure why you have +1 in min_order = fls(ac->ac_o_ex.fe_len) + 1.

> +
> +		if (num_stripe_clusters > 0) {
> +			/*
> +			 * Try to round up the adjusted goal to stripe size
						        ^^^ goal length?

> +			 * (in cluster units) multiple for efficiency.
> +			 *
> +			 * XXX: Is s->stripe always a power of 2? In that case
> +			 * we can use the faster round_up() variant.
> +			 */

I don't think s->stripe has to be a power of 2. E.g. when you have three
data disks in a RAID config.

Otherwise the patch looks good to me.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR