On Sun, 20 Apr 2025 at 19:43, Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> On 4/20/25 03:53, Carlos Maiolino wrote:
> > 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/
>
> A possible local solution is below. Note the variable type change from s64 to u64.
>
> Guenter
> ---
>
> diff --git a/fs/xfs/xfs_zone_gc.c b/fs/xfs/xfs_zone_gc.c
> index 8c541ca71872..6dde2a680e75 100644
> --- a/fs/xfs/xfs_zone_gc.c
> +++ b/fs/xfs/xfs_zone_gc.c
> @@ -170,7 +170,7 @@ bool
> xfs_zoned_need_gc(
> struct xfs_mount *mp)
> {
> - s64 available, free;
> + u64 available, free, rem;
>
> if (!xfs_group_marked(mp, XG_TYPE_RTG, XFS_RTG_RECLAIMABLE))
> return false;
> @@ -183,7 +183,12 @@ xfs_zoned_need_gc(
> return true;
>
> free = xfs_estimate_freecounter(mp, XC_FREE_RTEXTENTS);
> - if (available < mult_frac(free, mp->m_zonegc_low_space, 100))
> +
> + rem = do_div(free, 100);
> + free = free * mp->m_zonegc_low_space +
> + div_u64(rem * mp->m_zonegc_low_space, 100);
> +
> + if (available < free)
> return true;
>
> return false;
There's also mul_u64_u32_div():
static inline u64 mul_u64_u32_div(u64 a, u32 mul, u32 divisor)
...
Unsigned, too.
Gr{oetje,eeting}s,
Geert
--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds
