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btree: Limited leaf discarding in mdir commit to shrub roots only

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What a mouthful.

The unconditional bshrub leaf discarding in lfs3_mdir_commit was copied
from the previous btree leaf caching implementation, but discarding
_all_ bshrub leaves on _every_ mdir commit is a bit insane.

Really, the only bshrub leaves that ever need to be discarded here are
the shrub roots, which are already questionable leaf caching targets
because they're already cached as the root rbyd.

An alternative option would be to just never cache shrub roots, but
tinkering around with the idea showed it would be more costly that
conditionally discarding leaves in lfs3_mdir_commit. At least here we
can reuse some of the logic that discards file leaves.

I'm also probably overthinking what is only a small code cost:

           code          stack          ctx
  before: 36784           2400          684
  after:  36792 (+0.0%)   2400 (+0.0%)  684 (+0.0%)

This doesn't take into account how much CPU time is spent creating rbyd
copies, but that is not something we are optimizing for.
This commit is contained in:
Christopher Haster
2025-07-20 12:33:49 -05:00
parent cd9f93d859
commit ae53c326d6
1 changed files with 20 additions and 18 deletions
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lfs3.c
+20 -18
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@@ -9422,27 +9422,29 @@ static int lfs3_mdir_commit(lfs3_t *lfs3, lfs3_mdir_t *mdir,
// update any staged bshrubs
for (lfs3_handle_t *h = lfs3->handles; h; h = h->next) {
// if we moved a shrub, we also need to discard any related
// leaves that moved
#ifndef LFS3_KVONLY
if (lfs3_o_type(h->flags) == LFS3_TYPE_REG
&& lfs3_bptr_block(&((lfs3_file_t*)h)->leaf.bptr)
== ((lfs3_bshrub_t*)h)->shrub.r.blocks[0]
&& ((lfs3_bshrub_t*)h)->shrub_.blocks[0]
!= ((lfs3_bshrub_t*)h)->shrub.r.blocks[0]) {
lfs3_file_discardleaf((lfs3_file_t*)h);
}
#endif
// update the shrub
if (lfs3_o_isbshrub(h->flags)) {
// if we moved a shrub, we also need to discard any leaves
// that moved
if (((lfs3_bshrub_t*)h)->shrub_.blocks[0]
!= ((lfs3_bshrub_t*)h)->shrub.r.blocks[0]) {
// discard any bshrub leaves that moved
if (((lfs3_bshrub_t*)h)->shrub.leaf.rbyd.blocks[0]
== ((lfs3_bshrub_t*)h)->shrub.r.blocks[0]) {
lfs3_bshrub_discardleaf((lfs3_bshrub_t*)h);
}
// discard any file leaves that moved
#ifndef LFS3_KVONLY
if (lfs3_o_type(h->flags) == LFS3_TYPE_REG
&& lfs3_bptr_block(&((lfs3_file_t*)h)->leaf.bptr)
== ((lfs3_bshrub_t*)h)->shrub.r.blocks[0]) {
lfs3_file_discardleaf((lfs3_file_t*)h);
}
#endif
}
((lfs3_bshrub_t*)h)->shrub.r = ((lfs3_bshrub_t*)h)->shrub_;
// TODO do we really need to discard all shrub leaves on
// every mdir commit? shouldn't we just not cache the root?
// (which is already cached!)
//
// discard any leaves that may have moved
lfs3_bshrub_discardleaf((lfs3_bshrub_t*)h);
}
}