linux-ext4 - Re: [RFC]Ext4: journal credits reservation fixes for DIO, fallocate and delalloc writepages

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Date:	Wed, 23 Jul 2008 14:19:21 -0400
From:	Theodore Tso <tytso@....edu>
To:	Andreas Dilger <adilger@....com>
Cc:	Mingming Cao <cmm@...ibm.com>, linux-ext4@...r.kernel.org,
	Shehjar Tikoo <shehjart@....unsw.edu.au>
Subject: Re: [RFC]Ext4: journal credits reservation fixes for DIO,
	fallocate and delalloc writepages

On Tue, Jul 22, 2008 at 07:18:02PM -0600, Andreas Dilger wrote:
> On Jul 22, 2008  17:51 -0700, Mingming Cao wrote:
> > + * Calulate the total number of credits to reserve to fit
> > + * the modification of @num pages into a single transaction
> > + */
> > +int ext4_writepages_trans_blocks(struct inode *inode, int num)
> > +{
> > +	int bpp = ext4_journal_blocks_per_page(inode);
> > +	int nrblocks = num * bpp;
> > +
> > +	if (!EXT4_I(inode)->i_flags & EXT4_EXTENTS_FL)
> > +		return ext4_writepages_trans_blocks_old(inode, nrblocks);
> 
> This should be "if (!(EXT4_I(inode)->i_flags & EXT_EXTENTS_FL))", and
> we should probably make it "unlikely()" since we expect most new files
> in an ext4 filesystem are extent mapped.

The cost of unlikely() can be pretty bad; the rule of thumb I've heard
is that unless it's less than 1% vs. 99%, you should probably avoid
using likely() and unlikely().  Given that there will be a fair number
of people who will be doing upgrades of existing ext3 filesystems, I
don't think using unlikely() would be a good choice here.

      	    	  	     	      	     - Ted
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