[PATCH v2 2/4] fetch: use batched reference updates
- From
Karthik Nayak <karthik.188@gmail.com>
- Date
- May 15, 2025, 14:07 UTC
- Message-ID
- <20250515-501-update-git-fetch-1-to-use-partial-transactions-v2-2-80cbaaa55d2e@gmail.com>
- In-Reply-To
- <20250515-501-update-git-fetch-1-to-use-partial-transactions-v2-0-80cbaaa55d2e@gmail.com>
The reference updates performed as a part of 'git-fetch(1)', take place one at a time. For each reference update, a new transaction is created and committed. This is necessary to ensure we can allow individual updates to fail without failing the entire command. The command also supports an '--atomic' mode, which uses a single transaction to update all of the references. But this mode has an all-or-nothing approach, where if a single update fails, all updates would fail.
In 23fc8e4f61 (refs: implement batch reference update support, 2025-04-08), we introduced a new mechanism to batch reference updates. Under the hood, this uses a single transaction to perform a batch of reference updates, while allowing only individual updates to fail. Utilize this newly introduced batch update mechanism in 'git-fetch(1)'. This provides a significant bump in performance, especially when dealing with repositories with large number of references.
Adding support for batched updates is simply modifying the flow to also create a batch update transaction in the non-atomic flow.
With the reftable backend there is a 22x performance improvement, when performing 'git-fetch(1)' with 10000 refs:
Benchmark 1: fetch: many refs (refformat = reftable, refcount = 10000, revision = master)
Time (mean ± σ): 3.403 s ± 0.775 s [User: 1.875 s, System: 1.417 s]
Range (min … max): 2.454 s … 4.529 s 10 runs Benchmark 2: fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD)
Time (mean ± σ): 154.3 ms ± 17.6 ms [User: 102.5 ms, System: 56.1 ms]
Range (min … max): 145.2 ms … 220.5 ms 18 runs Summary
fetch: many refs (refformat = reftable, refcount = 10000, revision = HEAD) ran
22.06 ± 5.62 times faster than fetch: many refs (refformat = reftable, refcount = 10000, revision = master)In similar conditions, the files backend sees a 1.25x performance improvement:
Benchmark 1: fetch: many refs (refformat = files, refcount = 10000, revision = master)
Time (mean ± σ): 605.5 ms ± 9.4 ms [User: 117.8 ms, System: 483.3 ms]
Range (min … max): 595.6 ms … 621.5 ms 10 runs Benchmark 2: fetch: many refs (refformat = files, refcount = 10000, revision = HEAD)
Time (mean ± σ): 485.8 ms ± 4.3 ms [User: 91.1 ms, System: 396.7 ms]
Range (min … max): 477.6 ms … 494.3 ms 10 runs Summary
fetch: many refs (refformat = files, refcount = 10000, revision = HEAD) ran
1.25 ± 0.02 times faster than fetch: many refs (refformat = files, refcount = 10000, revision = master)With this we'll either be using a regular transaction or a batch update transaction. This helps cleanup some code which is no longer needed as we'll now always have some type of 'ref_transaction' object being propagated.
One big change is that earlier, each individual update would propagate a failure. Whereas now, the `ref_transaction_for_each_rejected_update` function is called at the end of the flow to capture the exit status for 'git-fetch(1)' and also to print F/D conflict errors. This does change the order of the errors being printed, but the behavior stays the same.
Since transaction errors are now explicitly defined as part of 76e760b999 (refs: introduce enum-based transaction error types, 2025-04-08), utilize them and get rid of custom errors defined within 'builtin/fetch.c'.
Signed-off-by: Karthik Nayak <karthik.188@gmail.com> --- builtin/fetch.c | 128 ++++++++++++++++++++++++++++++++------------------------ 1 file changed, 74 insertions(+), 54 deletions(-)
diff --git a/builtin/fetch.c b/builtin/fetch.c index 5279997c96..15eac2b1c2 100644 --- a/builtin/fetch.c +++ b/builtin/fetch.c @@ -641,9 +641,6 @@ static struct ref *get_ref_map(struct remote *remote, return ref_map; } -#define STORE_REF_ERROR_OTHER 1 -#define STORE_REF_ERROR_DF_CONFLICT 2 - static int s_update_ref(const char *action, struct ref *ref, struct ref_transaction *transaction, @@ -651,7 +648,6 @@ static int s_update_ref(const char *action, { char *msg; char *rla = getenv("GIT_REFLOG_ACTION"); - struct ref_transaction *our_transaction = NULL; struct strbuf err = STRBUF_INIT; int ret; @@ -661,43 +657,10 @@ static int s_update_ref(const char *action, rla = default_rla.buf; msg = xstrfmt("%s: %s", rla, action); - /* - * If no transaction was passed to us, we manage the transaction - * ourselves. Otherwise, we trust the caller to handle the transaction - * lifecycle. - */ - if (!transaction) { - transaction = our_transaction = ref_store_transaction_begin(get_main_ref_store(the_repository), - 0, &err); - if (!transaction) { - ret = STORE_REF_ERROR_OTHER; - goto out; - } - } - ret = ref_transaction_update(transaction, ref->name, &ref->new_oid, check_old ? &ref->old_oid : NULL, NULL, NULL, 0, msg, &err); - if (ret) { - ret = STORE_REF_ERROR_OTHER; - goto out; - } - - if (our_transaction) { - switch (ref_transaction_commit(our_transaction, &err)) { - case 0: - break; - case REF_TRANSACTION_ERROR_NAME_CONFLICT: - ret = STORE_REF_ERROR_DF_CONFLICT; - goto out; - default: - ret = STORE_REF_ERROR_OTHER; - goto out; - } - } -out: - ref_transaction_free(our_transaction); if (ret) error("%s", err.buf); strbuf_release(&err); @@ -1139,7 +1102,6 @@ N_("it took %.2f seconds to check forced updates; you can use\n" "to avoid this check\n"); static int store_updated_refs(struct display_state *display_state, - const char *remote_name, int connectivity_checked, struct ref_transaction *transaction, struct ref *ref_map, struct fetch_head *fetch_head, @@ -1277,11 +1239,6 @@ static int store_updated_refs(struct display_state *display_state, } } - if (rc & STORE_REF_ERROR_DF_CONFLICT) - error(_("some local refs could not be updated; try running\n" - " 'git remote prune %s' to remove any old, conflicting " - "branches"), remote_name); - if (advice_enabled(ADVICE_FETCH_SHOW_FORCED_UPDATES)) { if (!config->show_forced_updates) { warning(_(warn_show_forced_updates)); @@ -1366,9 +1323,8 @@ static int fetch_and_consume_refs(struct display_state *display_state, } trace2_region_enter("fetch", "consume_refs", the_repository); - ret = store_updated_refs(display_state, transport->remote->name, - connectivity_checked, transaction, ref_map, - fetch_head, config); + ret = store_updated_refs(display_state, connectivity_checked, + transaction, ref_map, fetch_head, config); trace2_region_leave("fetch", "consume_refs", the_repository); out: @@ -1688,6 +1644,37 @@ static int set_head(const struct ref *remote_refs, struct remote *remote) return result; } +struct ref_rejection_data { + int *retcode; + int conflict_msg_shown; + const char *remote_name; +}; + +static void ref_transaction_rejection_handler(const char *refname, + const struct object_id *old_oid UNUSED, + const struct object_id *new_oid UNUSED, + const char *old_target UNUSED, + const char *new_target UNUSED, + enum ref_transaction_error err, + void *cb_data) +{ + struct ref_rejection_data *data = (struct ref_rejection_data *)cb_data; + + if (err == REF_TRANSACTION_ERROR_NAME_CONFLICT && !data->conflict_msg_shown) { + error(_("some local refs could not be updated; try running\n" + " 'git remote prune %s' to remove any old, conflicting " + "branches"), data->remote_name); + data->conflict_msg_shown = 1; + } else { + char *reason = ref_transaction_error_msg(err); + + error(_("fetching ref %s failed: %s"), refname, reason); + free(reason); + } + + *data->retcode = 1; +} + static int do_fetch(struct transport *transport, struct refspec *rs, const struct fetch_config *config) @@ -1808,6 +1795,24 @@ static int do_fetch(struct transport *transport, retcode = 1; } + /* + * If not atomic, we can still use batched updates, which would be much + * more performant. We don't initiate the transaction before pruning, + * since pruning must be an independent step, to avoid F/D conflicts. + * + * TODO: if reference transactions gain logical conflict resolution, we + * can delete and create refs (with F/D conflicts) in the same transaction + * and this can be moved about the 'prune_refs()' block. + */ + if (!transaction) { + transaction = ref_store_transaction_begin(get_main_ref_store(the_repository), + REF_TRANSACTION_ALLOW_FAILURE, &err); + if (!transaction) { + retcode = -1; + goto cleanup; + } + } + if (fetch_and_consume_refs(&display_state, transport, transaction, ref_map, &fetch_head, config)) { retcode = 1; @@ -1839,16 +1844,31 @@ static int do_fetch(struct transport *transport, free_refs(tags_ref_map); } - if (transaction) { - if (retcode) - goto cleanup; + if (retcode) + goto cleanup; - retcode = ref_transaction_commit(transaction, &err); + retcode = ref_transaction_commit(transaction, &err); + if (retcode) { + /* + * Explicitly handle transaction cleanup to avoid + * aborting an already closed transaction. + */ + ref_transaction_free(transaction); + transaction = NULL; + goto cleanup; + } + + if (!atomic_fetch) { + struct ref_rejection_data data = { + .retcode = &retcode, + .conflict_msg_shown = 0, + .remote_name = transport->remote->name, + }; + + ref_transaction_for_each_rejected_update(transaction, + ref_transaction_rejection_handler, + &data); if (retcode) { - /* - * Explicitly handle transaction cleanup to avoid - * aborting an already closed transaction. - */ ref_transaction_free(transaction); transaction = NULL; goto cleanup;
-- 2.49.0