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Re: [PATCH 4/9] xfs: introduce background inode reclaim work

To: Alex Elder <aelder@xxxxxxx>
Subject: Re: [PATCH 4/9] xfs: introduce background inode reclaim work
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 8 Apr 2011 10:19:18 +1000
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1302210982.2576.600.camel@doink>
References: <1302141445-27457-1-git-send-email-david@xxxxxxxxxxxxx> <1302141445-27457-5-git-send-email-david@xxxxxxxxxxxxx> <1302210982.2576.600.camel@doink>
User-agent: Mutt/1.5.20 (2009-06-14)
On Thu, Apr 07, 2011 at 04:16:22PM -0500, Alex Elder wrote:
> On Thu, 2011-04-07 at 11:57 +1000, Dave Chinner wrote:
> > +   if (!(mp->m_super->s_flags & MS_ACTIVE))
> > +           return;
> > +
> > +   rcu_read_lock();
> > +   if (radix_tree_tagged(&mp->m_perag_tree, XFS_ICI_RECLAIM_TAG)) {
> > +           queue_delayed_work(xfs_syncd_wq, &mp->m_reclaim_work,
> > +                   msecs_to_jiffies(xfs_syncd_centisecs / 6 * 10));
> 
> Probably better to do the multiply before the divide here.
> (But whatever... it's heuristic.)

I always tend to divide before multiply to prevent the multiple from
overflowing before the divide is done. In this case the granularity
of xfs_syncd_centisecs is sufficient that the rounding error of the
divide is meaningless. ie. 30s = 3000.

FWIW, I changed this from:

        xfs_syncd_centisecs / 6 * msecs_to_jiffies(10)

because msecs_to_jiffies() has larger rounding problems. e.g. @
CONFIG_HZ=250, msecs_to_jiffies(10) = 3 which is actually 12ms. That
is, we want to sleep for 5s at a time, and the two different
calculations give:

New:
        msecs_to_jiffies(3000 / 6 * 10) = 5000 / 4 jiffies
                                        = 1250 jiffies
                                        = 5s
Old:
        3000 / 6 * msecs_to_jiffies(10) = 500 * 3 jiffies
                                        = 1500 jiffies
                                        = 6s

This 20% rounding error is the reason we've recently noticed xfssyncd
running every 36s rather than every 30s....

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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