linux-ext4 - Re: [PATCH] ext4: Adjust the layout of the ext4_inode

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Message-ID: <20240603152419.dklvgmf6m4akdyzf@quack3>
Date: Mon, 3 Jun 2024 17:24:19 +0200
From: Jan Kara <jack@...e.cz>
To: JunChao Sun <sunjunchao2870@...il.com>
Cc: Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org, tytso@....edu,
	adilger.kernel@...ger.ca
Subject: Re: [PATCH] ext4: Adjust the layout of the ext4_inode_info structure
 to save memory.

On Mon 03-06-24 22:43:16, JunChao Sun wrote:
> Jan Kara <jack@...e.cz> 于2024年6月3日周一 22:07写道：
> >
> > On Mon 03-06-24 21:15:24, Junchao Sun wrote:
> > > Using pahole, we can see that there are some padding holes
> > > in the current ext4_inode_info structure. Adjusting the
> > > layout of ext4_inode_info can reduce these holes,
> > > resulting in the size of the structure decreasing
> > > from 2424 bytes to 2408 bytes.
> >
> >
> > > But AFAICT this will save two holes 4 bytes each so only 8 bytes in total?
> > > Not 16?
> 
> Indeed it's 16.
> Consider the layout int a; hole 0; int b; hole 1; And then move int b
> to hole 0 position, this adjustment resulted in saving 8 bytes. There
> are two adjustments like this, so it's 16. And GDB confirmed this.

Aha, OK. Thanks for clarification!

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