linux-ext4 - Re: [RFC 08/11] ext4: Don't skip prefetching BLOCK

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Date:   Thu, 23 Mar 2023 11:57:10 +0100
From:   Jan Kara <jack@...e.cz>
To:     Ojaswin Mujoo <ojaswin@...ux.ibm.com>
Cc:     Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
        Theodore Ts'o <tytso@....edu>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Ritesh Harjani <ritesh.list@...il.com>,
        Andreas Dilger <adilger@...ger.ca>
Subject: Re: [RFC 08/11] ext4: Don't skip prefetching BLOCK_UNINIT groups

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@...ux.ibm.com>
> > > Reviewed-by: Ritesh Harjani (IBM) <ritesh.list@...il.com>
> > 
> > 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@...e.cz>
> > 
> > 								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.

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