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

[Guenter Roeck <linux@roeck-us.net>]

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!

Guenter