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Investigated weights in rbyd's fetch, ended up with just cleanup

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I was hopeful it would be possible to remove the weight lookup in
lfsr_btree_namelookup. We do a linear search during fetch to find the
name and tag, so finding the weight as well for free looked promising.

Unfortunately, it seems to be impossible to reliably find the weight.

Consider what happens when we match an id that is later deleted. We know
the new id should be id-1, but we don't have enough information to
determine the new weight.

So just ended up adding a comment explaining the limitation and cleaning
up the logic in lfsr_rbyd_fetch a bit.
This commit is contained in:
Christopher Haster
2023-04-11 16:13:24 -05:00
parent cfaeeaa690
commit f878f3f03c
1 changed files with 21 additions and 20 deletions
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lfs.c
+21 -20
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@@ -835,12 +835,13 @@ typedef struct lfsr_find {
const char *name;
lfs_size_t name_size;
// if found, the tag/id will be placed in found_tag/found_id, otherwise
// found_tag will be zero and found_id will be set to where to insert
lfsr_tag_t predicted_tag;
lfsr_tag_t found_tag;
// if found, the tag/id will be placed in found_tag/found_id,
// otherwise found_tag will be zero and found_id will be set to
// the largest, smaller id (a good place to insert)
lfs_ssize_t predicted_id;
lfs_ssize_t found_id;
lfsr_tag_t predicted_tag;
lfsr_tag_t found_tag;
} lfsr_find_t;
@@ -1256,10 +1257,10 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
lfsr_find_t *find) {
// clear any previous state in our find
if (find) {
find->predicted_tag = 0;
find->predicted_id = -1;
find->found_tag = 0;
find->predicted_tag = 0;
find->found_id = -1;
find->found_tag = 0;
}
// read the revision count and get the crc started
@@ -1383,8 +1384,8 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
rbyd->weight = weight;
if (find) {
find->found_tag = find->predicted_tag;
find->found_id = find->predicted_id;
find->found_tag = find->predicted_tag;
}
}
@@ -1415,22 +1416,20 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
} else {
lfs_ssize_t delta = (lower+upper+w) - weight;
weight = lower+upper+w;
lfs_ssize_t id = lower+w-1;
// adjust any pending finds
if (find && find->predicted_id >= (lfs_ssize_t)lower) {
// pending find removed?
if (delta < 0
&& find->predicted_id
< (lfs_ssize_t)lower + -delta) {
find->predicted_tag = 0;
// TODO is this correct for insertion behavior?
if (find->predicted_id + delta < (lfs_ssize_t)lower) {
find->predicted_id = lower-1;
find->predicted_tag = 0;
} else {
find->predicted_id += delta;
}
}
// found our find?
// found our find request?
if (find && lfsr_tag_suptype(tag) == LFSR_TAG_NAME) {
// compare with disk
lfs_size_t d = lfs_min(size, find->name_size);
@@ -1450,14 +1449,13 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
}
// found match?
lfs_ssize_t id = lower+w-1;
if (cmp == LFS_CMP_EQ) {
find->predicted_tag = tag;
find->predicted_id = id;
find->predicted_tag = tag;
// didn't find a match, but found a better insertion point
} else if (cmp == LFS_CMP_LT && id > find->predicted_id) {
find->predicted_tag = 0;
find->predicted_id = id;
find->predicted_tag = 0;
}
}
@@ -1484,9 +1482,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
// need a new erase
uint32_t fcrc_ = 0;
int err = lfs_bd_crc32c(lfs,
// TODO is this the correct hint?
NULL, &lfs->rcache, lfs->cfg->block_size
- lfs_min(off, lfs->cfg->block_size),
NULL, &lfs->rcache, fcrc.size,
rbyd->block, rbyd->off, fcrc.size, &fcrc_);
if (err && err != LFS_ERR_CORRUPT) {
return err;
@@ -2969,7 +2965,12 @@ static lfs_ssize_t lfsr_btree_namelookup(lfs_t *lfs,
// the find may not match exactly, but it will indicate which id we
// should follow
// TODO can we actually get weight in find?
//
// Note that we can't reliably find the weight in fetch. If, during our
// linear search, we match an id that is later deleted, we know id-1
// should be the new id, but we don't have enough information to
// determine the new weight. So unfortunately we need an additional
// lookup to find the weight.
lfsr_tag_t tag__;
lfs_ssize_t rid__;
lfs_size_t weight__;