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btrfs: increase the metadata allowance for the free_space_cache

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Currently, there is no way for the free space cache to recover from
being serviced by purely bitmaps because the extent threshold is set to
0 in recalculate_thresholds() when we surpass the metadata allowance.

This adds a recovery mechanism by keeping large extents out of the
bitmaps and increases the metadata upper bound to 64KB. The recovery
mechanism bypasses this upper bound, thus making it a soft upper bound.
But, with the bypass being 1MB or greater, it shouldn't add unbounded
overhead.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Dennis Zhou <dennis@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Dennis Zhou 2020-01-02 16:26:43 -05:00 committed by David Sterba
parent dbc2a8c927
commit 5d90c5c757
1 changed files with 11 additions and 15 deletions
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26
fs/btrfs/free-space-cache.c
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@ -24,7 +24,8 @@
#include "discard.h"
#define BITS_PER_BITMAP (PAGE_SIZE * 8UL)
#define MAX_CACHE_BYTES_PER_GIG SZ_32K
#define MAX_CACHE_BYTES_PER_GIG SZ_64K
#define FORCE_EXTENT_THRESHOLD SZ_1M
struct btrfs_trim_range {
u64 start;
@ -1694,26 +1695,17 @@ static void recalculate_thresholds(struct btrfs_free_space_ctl *ctl)
ASSERT(ctl->total_bitmaps <= max_bitmaps);
/*
* The goal is to keep the total amount of memory used per 1gb of space
* at or below 32k, so we need to adjust how much memory we allow to be
* used by extent based free space tracking
* We are trying to keep the total amount of memory used per 1GiB of
* space to be MAX_CACHE_BYTES_PER_GIG. However, with a reclamation
* mechanism of pulling extents >= FORCE_EXTENT_THRESHOLD out of
* bitmaps, we may end up using more memory than this.
*/
if (size < SZ_1G)
max_bytes = MAX_CACHE_BYTES_PER_GIG;
else
max_bytes = MAX_CACHE_BYTES_PER_GIG * div_u64(size, SZ_1G);
/*
* we want to account for 1 more bitmap than what we have so we can make
* sure we don't go over our overall goal of MAX_CACHE_BYTES_PER_GIG as
* we add more bitmaps.
*/
bitmap_bytes = (ctl->total_bitmaps + 1) * ctl->unit;
if (bitmap_bytes >= max_bytes) {
ctl->extents_thresh = 0;
return;
}
bitmap_bytes = ctl->total_bitmaps * ctl->unit;
/*
* we want the extent entry threshold to always be at most 1/2 the max
@ -2099,6 +2091,10 @@ static bool use_bitmap(struct btrfs_free_space_ctl *ctl,
forced = true;
#endif
/* This is a way to reclaim large regions from the bitmaps. */
if (!forced && info->bytes >= FORCE_EXTENT_THRESHOLD)
return false;
/*
* If we are below the extents threshold then we can add this as an
* extent, and don't have to deal with the bitmap