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@xxxxxxxxxx/
