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;
