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authorMichael Haggerty <mhagger@alum.mit.edu>2017-10-28 09:16:02 (GMT)
committerJunio C Hamano <gitster@pobox.com>2017-10-30 00:45:15 (GMT)
commit7c6bd25c7d65e777b5bb26ad5c8248fb3607e6d5 (patch)
treed2801cff3122ec0eca892bad37040c22e20fa0ba /refs/files-backend.c
parentcf79bd9f4c34c635a5a7eeff2699a24744c5a129 (diff)
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files-backend: don't rewrite the `packed-refs` file unnecessarily
Even when we are deleting references, we needn't overwrite the `packed-refs` file if the references that we are deleting only exist as loose references. Implement this optimization as follows: * Add a function `is_packed_transaction_needed()`, which checks whether a given packed-refs transaction actually needs to be carried out (i.e., it returns false if the transaction obviously wouldn't have any effect). This function must be called while holding the `packed-refs` lock to avoid races. * Change `files_transaction_prepare()` to check whether the packed-refs transaction is actually needed. If not, squelch it, but continue holding the `packed-refs` lock until the end of the transaction to avoid races. This fixes a mild regression caused by dc39e09942 (files_ref_store: use a transaction to update packed refs, 2017-09-08). Before that commit, unnecessary rewrites of `packed-refs` were suppressed by `repack_without_refs()`. But the transaction-based writing introduced by that commit didn't perform that optimization. Note that the pre-dc39e09942 code still had to *read* the whole `packed-refs` file to determine that the rewrite could be skipped, so the performance for the cases that the write could be elided was `O(N)` in the number of packed references both before and after dc39e09942. But after that commit the constant factor increased. This commit reimplements the optimization of eliding unnecessary `packed-refs` rewrites. That, plus the fact that since cfa2e29c34 (packed_ref_store: get rid of the `ref_cache` entirely, 2017-03-17) we don't necessarily have to read the whole `packed-refs` file at all, means that deletes of one or a few loose references can now be done with `O(n lg N)` effort, where `n` is the number of loose references being deleted and `N` is the total number of packed references. This commit fixes two tests in t1409. Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'refs/files-backend.c')
-rw-r--r--refs/files-backend.c18
1 files changed, 17 insertions, 1 deletions
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 961424a..da8a986 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2562,7 +2562,23 @@ static int files_transaction_prepare(struct ref_store *ref_store,
goto cleanup;
}
backend_data->packed_refs_locked = 1;
- ret = ref_transaction_prepare(packed_transaction, err);
+
+ if (is_packed_transaction_needed(refs->packed_ref_store,
+ packed_transaction)) {
+ ret = ref_transaction_prepare(packed_transaction, err);
+ } else {
+ /*
+ * We can skip rewriting the `packed-refs`
+ * file. But we do need to leave it locked, so
+ * that somebody else doesn't pack a reference
+ * that we are trying to delete.
+ */
+ if (ref_transaction_abort(packed_transaction, err)) {
+ ret = TRANSACTION_GENERIC_ERROR;
+ goto cleanup;
+ }
+ backend_data->packed_transaction = NULL;
+ }
}
cleanup: