Re: [RFC 08/11] ext4: Don't skip prefetching BLOCK_UNINIT groups

[Ojaswin Mujoo]

On Thu, Mar 23, 2023 at 11:57:10AM +0100, Jan Kara wrote:
> On Fri 17-03-23 16:25:04, Ojaswin Mujoo wrote:
> > On Thu, Mar 09, 2023 at 03:14:22PM +0100, Jan Kara wrote:
> > > On Fri 27-01-23 18:07:35, Ojaswin Mujoo wrote:
> > > > Currently, ext4_mb_prefetch() and ext4_mb_prefetch_fini() skip
> > > > BLOCK_UNINIT groups since fetching their bitmaps doesn't need disk IO.
> > > > As a consequence, we end not initializing the buddy structures and CR0/1
> > > > lists for these BGs, even though it can be done without any disk IO
> > > > overhead. Hence, don't skip such BGs during prefetch and prefetch_fini.
> > > > 
> > > > This improves the accuracy of CR0/1 allocation as earlier, we could have
> > > > essentially empty BLOCK_UNINIT groups being ignored by CR0/1 due to their buddy
> > > > not being initialized, leading to slower CR2 allocations. With this patch CR0/1
> > > > will be able to discover these groups as well, thus improving performance.
> > > > 
> > > > Signed-off-by: Ojaswin Mujoo <ojaswin@xxxxxxxxxxxxx>
> > > > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@xxxxxxxxx>
> > > 
> > > The patch looks good. I just somewhat wonder - this change may result in
> > > uninitialized groups being initialized and used earlier (previously we'd
> > > rather search in other already initialized groups) which may spread
> > > allocations more. But I suppose that's fine and uninit groups are not
> > > really a feature meant to limit fragmentation and as the filesystem ages
> > > the differences should be minimal. So feel free to add:
> > > 
> > > Reviewed-by: Jan Kara <jack@xxxxxxx>
> > > 
> > > 								Honza
> > Thanks for the review. As for the allocation spread, I agree that it
> > should be something our goal determination logic should take care of
> > rather than limiting the BGs available to the allocator.
> > 
> > Another point I wanted to discuss wrt this patch series was why were the
> > BLOCK_UNINIT groups not being prefetched earlier. One point I can think
> > of is that this might lead to memory pressure when we have too many
> > empty BGs in a very large (say terabytes) disk.
> > 
> > But i'd still like to know if there's some history behind not
> > prefetching block uninit.
> 
> Hum, I don't remember anything. Maybe Ted will. You can ask him today on a
> call.
Unfortunately, couldn't join it last time :) I'll check with him on
upcoming Thurs.

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