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

[Jan Kara]

On Fri 17-03-23 17:07:21, Ojaswin Mujoo wrote:
> On Thu, Mar 09, 2023 at 04:06:49PM +0100, Jan Kara wrote:
> > On Fri 27-01-23 18:07:38, Ojaswin Mujoo wrote:
> > > +/*
> > > + * 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...
> 
> Yes that is possible. In ext4_mb_normalize_request, for orignal request len <
> 8MB we actually determine the goal length based on the length of the
> file (i_size) rather than the length of the original request. For eg:
> 
> 	if (size <= 16 * 1024) {
> 		size = 16 * 1024;
> 	} else if (size <= 32 * 1024) {
> 		size = 32 * 1024;
> 	} else if (size <= 64 * 1024) {
> 		size = 64 * 1024;
> 
> and this goes all the way upto size = 8MB. So for a case where the file
> is >8MB, even if the original len is of 1 block(4KB), the goal len would
> be of 2048 blocks(8MB). That's why we decided to add a tunable depending
> on the user's preference.

Ah, I see. The problem with these tunables is that nobody knows to which
value tune them :). But yeah, the default value looks sane so I don't
object.

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