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

Ojaswin Mujoo
2023-03-25T14:47:08Z

On Thu, Mar 23, 2023 at 12:05:32PM +0100, Jan Kara wrote:
> 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.
>
Right, so in our workloads we were kinda seeing good improvement at this
value.

But I think it really depends on how fragmented the FS is, we picked
trim order 3 as a safe value so we don't end up trimming too much when
CR2 could go and find something better.

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