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Date: Tue, 1 May 2018 17:39:45 +0200
From: David Sterba <dsterba@suse.cz>
To: Timofey Titovets <nefelim4ag@gmail.com>
Cc: David Sterba <dsterba@suse.cz>, linux-btrfs <linux-btrfs@vger.kernel.org>
Subject: Re: [PATCH 4/4] [RESEND] Btrfs: reduce size of struct btrfs_inode
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On Sat, Apr 28, 2018 at 10:01:57AM +0000, Timofey Titovets wrote:
> May be i misunderstood something, but i was think that slab combine
> several pages in continuous range, so object in slab can cross page
> boundary.  So, all calculation will be very depends on scale of slab
> size.
> 
> i.e. on my machine that looks quite different:
> name                    <active_objs>  <num_objs>  <objsize>  <objperslab>   <pagesperslab>
> btrfs_inode             142475         146272      1136       28             8
> 
> So,
> PAGE_SIZE * pagesperslab / objperslab
> 4096 * 8 / 28 = 1170.28
> 
> 4096*8 - 1136*28 = 960
> 
> That's looks like object can cross page boundary in slab.
> So, if size reduced to 1128,
> 4096 * 8 / 29 = 1129.93
> 4096*8 - 1128*29 = 56
> 
> Did i miss something?

Then this depends on the allocator, SLAB does 4k pages and SLUB 32k, so
there's a value in reducing the inode size.