linux-kernel - Re: [PATCH] xfs: add tunable threshold parameter for triggering zone GC

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Message-ID: <mt3tlttnxheypljdkyy6bpjfkb7n5pm2w35wuf7bsma3btwnua@a3zljdrqqaq7>
Date: Sun, 20 Apr 2025 12:53:22 +0200
From: Carlos Maiolino <cem@...nel.org>
To: Guenter Roeck <linux@...ck-us.net>
Cc: Hans Holmberg <Hans.Holmberg@....com>, 
	Dave Chinner <david@...morbit.com>, "Darrick J . Wong" <djwong@...nel.org>, hch <hch@....de>, 
	"linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xfs: add tunable threshold parameter for triggering zone
 GC

On Sun, Apr 20, 2025 at 02:47:02AM -0700, Guenter Roeck wrote:
> On Tue, Mar 25, 2025 at 09:10:49AM +0000, Hans Holmberg wrote:
> > Presently we start garbage collection late - when we start running
> > out of free zones to backfill max_open_zones. This is a reasonable
> > default as it minimizes write amplification. The longer we wait,
> > the more blocks are invalidated and reclaim cost less in terms
> > of blocks to relocate.
> >
> > Starting this late however introduces a risk of GC being outcompeted
> > by user writes. If GC can't keep up, user writes will be forced to
> > wait for free zones with high tail latencies as a result.
> >
> > This is not a problem under normal circumstances, but if fragmentation
> > is bad and user write pressure is high (multiple full-throttle
> > writers) we will "bottom out" of free zones.
> >
> > To mitigate this, introduce a zonegc_low_space tunable that lets the
> > user specify a percentage of how much of the unused space that GC
> > should keep available for writing. A high value will reclaim more of
> > the space occupied by unused blocks, creating a larger buffer against
> > write bursts.
> >
> > This comes at a cost as write amplification is increased. To
> > illustrate this using a sample workload, setting zonegc_low_space to
> > 60% avoids high (500ms) max latencies while increasing write
> > amplification by 15%.
> >
> ...
> >  bool
> >  xfs_zoned_need_gc(
> >  	struct xfs_mount	*mp)
> >  {
> > +	s64			available, free;
> > +
> ...
> > +
> > +	free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);
> > +	if (available < mult_frac(free, mp->m_zonegc_low_space, 100))
> > +		return true;
> > +
> 
> With some 32-bit builds (parisc, openrisc so far):
> 
> Error log:
> ERROR: modpost: "__divdi3" [fs/xfs/xfs.ko] undefined!
> ERROR: modpost: "__umoddi3" [fs/xfs/xfs.ko] undefined!
> ERROR: modpost: "__moddi3" [fs/xfs/xfs.ko] undefined!
> ERROR: modpost: "__udivdi3" [fs/xfs/xfs.ko] undefined!
> 

I opened a discussion about this:

https://lore.kernel.org/lkml/20250419115157.567249-1-cem@kernel.org/