Extended lfs_fs_gc to compact metadata, compact_thresh

This extends lfs_fs_gc to now handle three things:

1. Calls mkconsistent if not already consistent
2. Compacts metadata > compact_thresh
3. Populates the block allocator

Which should be all of the janitorial work that can be done without
additional on-disk data structures.

Normally, metadata compaction occurs when an mdir is full, and results in
mdirs that are at most block_size/2.

Now, if you call lfs_fs_gc, littlefs will eagerly compact any mdirs that
exceed the compact_thresh configuration option. Because the resulting
mdirs are at most block_size/2, it only makes sense for compact_thresh to
be >= block_size/2 and <= block_size.

Additionally, there are some special values:

- compact_thresh=0  => defaults to ~88% block_size, may change
- compact_thresh=-1 => disables metadata compaction during lfs_fs_gc

Note that compact_thresh only affects lfs_fs_gc. Normal compactions
still only occur when full.

lfs.c

@@ -593,19 +593,6 @@ static int lfs_rawunmount(lfs_t *lfs);
 
 
 /// Block allocator ///
-#ifndef LFS_READONLY
-static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
-    lfs_t *lfs = (lfs_t*)p;
-    lfs_block_t off = ((block - lfs->lookahead.start)
-            + lfs->block_count) % lfs->block_count;
-
-    if (off < lfs->lookahead.size) {
-        lfs->lookahead.buffer[off / 8] |= 1U << (off % 8);
-    }
-
-    return 0;
-}
-#endif
 
 // allocations should call this when all allocated blocks are committed to
 // the filesystem
@@ -624,7 +611,21 @@ static void lfs_alloc_drop(lfs_t *lfs) {
 }
 
 #ifndef LFS_READONLY
-static int lfs_fs_rawgc(lfs_t *lfs) {
+static int lfs_alloc_lookahead(void *p, lfs_block_t block) {
+    lfs_t *lfs = (lfs_t*)p;
+    lfs_block_t off = ((block - lfs->lookahead.start)
+            + lfs->block_count) % lfs->block_count;
+
+    if (off < lfs->lookahead.size) {
+        lfs->lookahead.buffer[off / 8] |= 1U << (off % 8);
+    }
+
+    return 0;
+}
+#endif
+
+#ifndef LFS_READONLY
+static int lfs_alloc_scan(lfs_t *lfs) {
     // move lookahead buffer to the first unused block
     //
     // note we limit the lookahead buffer to at most the amount of blocks
@@ -693,7 +694,7 @@ static int lfs_alloc(lfs_t *lfs, lfs_block_t *block) {
 
         // No blocks in our lookahead buffer, we need to scan the filesystem for
         // unused blocks in the next lookahead window.
-        int err = lfs_fs_rawgc(lfs);
+        int err = lfs_alloc_scan(lfs);
         if(err) {
             return err;
         }
@@ -4172,6 +4173,14 @@ static int lfs_init(lfs_t *lfs, const struct lfs_config *cfg) {
     // wear-leveling.
     LFS_ASSERT(lfs->cfg->block_cycles != 0);
 
+    // check that compact_thresh makes sense
+    //
+    // metadata can't be compacted below block_size/2, and metadata can't
+    // exceed a block_size
+    LFS_ASSERT(lfs->cfg->compact_thresh == 0
+            || lfs->cfg->compact_thresh >= lfs->cfg->block_size/2);
+    LFS_ASSERT(lfs->cfg->compact_thresh == (lfs_size_t)-1
+            || lfs->cfg->compact_thresh <= lfs->cfg->block_size);
 
     // setup read cache
     if (lfs->cfg->read_buffer) {
@@ -5063,6 +5072,57 @@ static lfs_ssize_t lfs_fs_rawsize(lfs_t *lfs) {
     return size;
 }
 
+// explicit garbage collection
+#ifndef LFS_READONLY
+static int lfs_fs_rawgc(lfs_t *lfs) {
+    // force consistency, even if we're not necessarily going to write,
+    // because this function is supposed to take care of janitorial work
+    // isn't it?
+    int err = lfs_fs_forceconsistency(lfs);
+    if (err) {
+        return err;
+    }
+
+    // try to compact metadata pairs, note we can't really accomplish
+    // anything if compact_thresh doesn't at least leave a prog_size
+    // available
+    if (lfs->cfg->compact_thresh
+            < lfs->cfg->block_size - lfs->cfg->prog_size) {
+        // iterate over all mdirs
+        lfs_mdir_t mdir = {.tail = {0, 1}};
+        while (!lfs_pair_isnull(mdir.tail)) {
+            err = lfs_dir_fetch(lfs, &mdir, mdir.tail);
+            if (err) {
+                return err;
+            }
+
+            // not erased? exceeds our compaction threshold?
+            if (!mdir.erased || ((lfs->cfg->compact_thresh == 0)
+                    ? mdir.off > lfs->cfg->block_size - lfs->cfg->block_size/8
+                    : mdir.off > lfs->cfg->compact_thresh)) {
+                // the easiest way to trigger a compaction is to mark
+                // the mdir as unerased and add an empty commit
+                mdir.erased = false;
+                err = lfs_dir_commit(lfs, &mdir, NULL, 0);
+                if (err) {
+                    return err;
+                }
+            }
+        }
+    }
+
+    // try to populate the lookahead buffer, unless it's already full
+    if (lfs->lookahead.size < 8*lfs->cfg->lookahead_size) {
+        err = lfs_alloc_scan(lfs);
+        if (err) {
+            return err;
+        }
+    }
+
+    return 0;
+}
+#endif
+
 #ifndef LFS_READONLY
 static int lfs_fs_rawgrow(lfs_t *lfs, lfs_size_t block_count) {
     // shrinking is not supported
@@ -6269,32 +6329,32 @@ int lfs_fs_traverse(lfs_t *lfs, int (*cb)(void *, lfs_block_t), void *data) {
 }
 
 #ifndef LFS_READONLY
-int lfs_fs_gc(lfs_t *lfs) {
+int lfs_fs_mkconsistent(lfs_t *lfs) {
     int err = LFS_LOCK(lfs->cfg);
     if (err) {
         return err;
     }
-    LFS_TRACE("lfs_fs_gc(%p)", (void*)lfs);
+    LFS_TRACE("lfs_fs_mkconsistent(%p)", (void*)lfs);
 
-    err = lfs_fs_rawgc(lfs);
+    err = lfs_fs_rawmkconsistent(lfs);
 
-    LFS_TRACE("lfs_fs_gc -> %d", err);
+    LFS_TRACE("lfs_fs_mkconsistent -> %d", err);
     LFS_UNLOCK(lfs->cfg);
     return err;
 }
 #endif
 
 #ifndef LFS_READONLY
-int lfs_fs_mkconsistent(lfs_t *lfs) {
+int lfs_fs_gc(lfs_t *lfs) {
     int err = LFS_LOCK(lfs->cfg);
     if (err) {
         return err;
     }
-    LFS_TRACE("lfs_fs_mkconsistent(%p)", (void*)lfs);
+    LFS_TRACE("lfs_fs_gc(%p)", (void*)lfs);
 
-    err = lfs_fs_rawmkconsistent(lfs);
+    err = lfs_fs_rawgc(lfs);
 
-    LFS_TRACE("lfs_fs_mkconsistent -> %d", err);
+    LFS_TRACE("lfs_fs_gc -> %d", err);
     LFS_UNLOCK(lfs->cfg);
     return err;
 }

lfs.h

@@ -229,6 +229,17 @@ struct lfs_config {
     // can track 8 blocks.
     lfs_size_t lookahead_size;
 
+    // Threshold for metadata compaction during lfs_fs_gc in bytes. Metadata
+    // pairs that exceed this threshold will be compacted during lfs_fs_gc.
+    // Defaults to ~88% block_size when zero, though the default may change
+    // in the future.
+    //
+    // Note this only affects lfs_fs_gc. Normal compactions still only occur
+    // when full.
+    //
+    // Set to -1 to disable metadata compaction during lfs_fs_gc.
+    lfs_size_t compact_thresh;
+
     // Optional statically allocated read buffer. Must be cache_size.
     // By default lfs_malloc is used to allocate this buffer.
     void *read_buffer;
@@ -711,18 +722,6 @@ lfs_ssize_t lfs_fs_size(lfs_t *lfs);
 // Returns a negative error code on failure.
 int lfs_fs_traverse(lfs_t *lfs, int (*cb)(void*, lfs_block_t), void *data);
 
-// Attempt to proactively find free blocks
-//
-// Calling this function is not required, but may allowing the offloading of
-// the expensive block allocation scan to a less time-critical code path.
-//
-// Note: littlefs currently does not persist any found free blocks to disk.
-// This may change in the future.
-//
-// Returns a negative error code on failure. Finding no free blocks is
-// not an error.
-int lfs_fs_gc(lfs_t *lfs);
-
 #ifndef LFS_READONLY
 // Attempt to make the filesystem consistent and ready for writing
 //
@@ -735,6 +734,24 @@ int lfs_fs_gc(lfs_t *lfs);
 int lfs_fs_mkconsistent(lfs_t *lfs);
 #endif
 
+#ifndef LFS_READONLY
+// Attempt any janitorial work
+//
+// This currently:
+// 1. Calls mkconsistent if not already consistent
+// 2. Compacts metadata > compact_thresh
+// 3. Populates the block allocator
+//
+// Though additional janitorial work may be added in the future.
+//
+// Calling this function is not required, but may allow the offloading of
+// expensive janitorial work to a less time-critical code path.
+//
+// Returns a negative error code on failure. Accomplishing nothing is not
+// an error.
+int lfs_fs_gc(lfs_t *lfs);
+#endif
+
 #ifndef LFS_READONLY
 // Grows the filesystem to a new size, updating the superblock with the new
 // block count.

runners/bench_runner.c

@@ -1321,6 +1321,7 @@ void perm_run(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     };
 
     struct lfs_emubd_config bdcfg = {

runners/bench_runner.h

@@ -95,11 +95,12 @@ intmax_t bench_define(size_t define);
 #define BLOCK_COUNT_i        5
 #define CACHE_SIZE_i         6
 #define LOOKAHEAD_SIZE_i     7
-#define BLOCK_CYCLES_i       8
-#define ERASE_VALUE_i        9
-#define ERASE_CYCLES_i       10
-#define BADBLOCK_BEHAVIOR_i  11
-#define POWERLOSS_BEHAVIOR_i 12
+#define COMPACT_THRESH_i     8
+#define BLOCK_CYCLES_i       9
+#define ERASE_VALUE_i        10
+#define ERASE_CYCLES_i       11
+#define BADBLOCK_BEHAVIOR_i  12
+#define POWERLOSS_BEHAVIOR_i 13
 
 #define READ_SIZE           bench_define(READ_SIZE_i)
 #define PROG_SIZE           bench_define(PROG_SIZE_i)
@@ -109,6 +110,7 @@ intmax_t bench_define(size_t define);
 #define BLOCK_COUNT         bench_define(BLOCK_COUNT_i)
 #define CACHE_SIZE          bench_define(CACHE_SIZE_i)
 #define LOOKAHEAD_SIZE      bench_define(LOOKAHEAD_SIZE_i)
+#define COMPACT_THRESH      bench_define(COMPACT_THRESH_i)
 #define BLOCK_CYCLES        bench_define(BLOCK_CYCLES_i)
 #define ERASE_VALUE         bench_define(ERASE_VALUE_i)
 #define ERASE_CYCLES        bench_define(ERASE_CYCLES_i)
@@ -124,6 +126,7 @@ intmax_t bench_define(size_t define);
     BENCH_DEF(BLOCK_COUNT,        ERASE_COUNT/lfs_max(BLOCK_SIZE/ERASE_SIZE,1))\
     BENCH_DEF(CACHE_SIZE,         lfs_max(64,lfs_max(READ_SIZE,PROG_SIZE))) \
     BENCH_DEF(LOOKAHEAD_SIZE,     16) \
+    BENCH_DEF(COMPACT_THRESH,     0) \
     BENCH_DEF(BLOCK_CYCLES,       -1) \
     BENCH_DEF(ERASE_VALUE,        0xff) \
     BENCH_DEF(ERASE_CYCLES,       0) \
@@ -131,7 +134,7 @@ intmax_t bench_define(size_t define);
     BENCH_DEF(POWERLOSS_BEHAVIOR, LFS_EMUBD_POWERLOSS_NOOP)
 
 #define BENCH_GEOMETRY_DEFINE_COUNT 4
-#define BENCH_IMPLICIT_DEFINE_COUNT 13
+#define BENCH_IMPLICIT_DEFINE_COUNT 14
 
 
 #endif

runners/test_runner.c

@@ -1346,6 +1346,7 @@ static void run_powerloss_none(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     #ifdef LFS_MULTIVERSION
         .disk_version       = DISK_VERSION,
     #endif
@@ -1422,6 +1423,7 @@ static void run_powerloss_linear(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     #ifdef LFS_MULTIVERSION
         .disk_version       = DISK_VERSION,
     #endif
@@ -1515,6 +1517,7 @@ static void run_powerloss_log(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     #ifdef LFS_MULTIVERSION
         .disk_version       = DISK_VERSION,
     #endif
@@ -1606,6 +1609,7 @@ static void run_powerloss_cycles(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     #ifdef LFS_MULTIVERSION
         .disk_version       = DISK_VERSION,
     #endif
@@ -1795,6 +1799,7 @@ static void run_powerloss_exhaustive(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     #ifdef LFS_MULTIVERSION
         .disk_version       = DISK_VERSION,
     #endif

runners/test_runner.h

@@ -88,12 +88,13 @@ intmax_t test_define(size_t define);
 #define BLOCK_COUNT_i        5
 #define CACHE_SIZE_i         6
 #define LOOKAHEAD_SIZE_i     7
-#define BLOCK_CYCLES_i       8
-#define ERASE_VALUE_i        9
-#define ERASE_CYCLES_i       10
-#define BADBLOCK_BEHAVIOR_i  11
-#define POWERLOSS_BEHAVIOR_i 12
-#define DISK_VERSION_i       13
+#define COMPACT_THRESH_i     8
+#define BLOCK_CYCLES_i       9
+#define ERASE_VALUE_i        10
+#define ERASE_CYCLES_i       11
+#define BADBLOCK_BEHAVIOR_i  12
+#define POWERLOSS_BEHAVIOR_i 13
+#define DISK_VERSION_i       14
 
 #define READ_SIZE           TEST_DEFINE(READ_SIZE_i)
 #define PROG_SIZE           TEST_DEFINE(PROG_SIZE_i)
@@ -103,6 +104,7 @@ intmax_t test_define(size_t define);
 #define BLOCK_COUNT         TEST_DEFINE(BLOCK_COUNT_i)
 #define CACHE_SIZE          TEST_DEFINE(CACHE_SIZE_i)
 #define LOOKAHEAD_SIZE      TEST_DEFINE(LOOKAHEAD_SIZE_i)
+#define COMPACT_THRESH      TEST_DEFINE(COMPACT_THRESH_i)
 #define BLOCK_CYCLES        TEST_DEFINE(BLOCK_CYCLES_i)
 #define ERASE_VALUE         TEST_DEFINE(ERASE_VALUE_i)
 #define ERASE_CYCLES        TEST_DEFINE(ERASE_CYCLES_i)
@@ -119,6 +121,7 @@ intmax_t test_define(size_t define);
     TEST_DEF(BLOCK_COUNT,        ERASE_COUNT/lfs_max(BLOCK_SIZE/ERASE_SIZE,1)) \
     TEST_DEF(CACHE_SIZE,         lfs_max(64,lfs_max(READ_SIZE,PROG_SIZE))) \
     TEST_DEF(LOOKAHEAD_SIZE,     16) \
+    TEST_DEF(COMPACT_THRESH,     0) \
     TEST_DEF(BLOCK_CYCLES,       -1) \
     TEST_DEF(ERASE_VALUE,        0xff) \
     TEST_DEF(ERASE_CYCLES,       0) \
@@ -127,7 +130,7 @@ intmax_t test_define(size_t define);
     TEST_DEF(DISK_VERSION,       0)
 
 #define TEST_GEOMETRY_DEFINE_COUNT 4
-#define TEST_IMPLICIT_DEFINE_COUNT 14
+#define TEST_IMPLICIT_DEFINE_COUNT 15
 
 
 #endif

tests/test_alloc.toml

@@ -7,6 +7,7 @@ if = 'BLOCK_CYCLES == -1'
 defines.FILES = 3
 defines.SIZE = '(((BLOCK_SIZE-8)*(BLOCK_COUNT-6)) / FILES)'
 defines.GC = [false, true]
+defines.COMPACT_THRESH = ['-1', '0', 'BLOCK_SIZE/2']
 code = '''
     const char *names[] = {"bacon", "eggs", "pancakes"};
     lfs_file_t files[FILES];
@@ -60,6 +61,7 @@ code = '''
 defines.FILES = 3
 defines.SIZE = '(((BLOCK_SIZE-8)*(BLOCK_COUNT-6)) / FILES)'
 defines.GC = [false, true]
+defines.COMPACT_THRESH = ['-1', '0', 'BLOCK_SIZE/2']
 code = '''
     const char *names[] = {"bacon", "eggs", "pancakes"};