Re: [PATCH v4 4/4] commit-graph: new filter ver. that fixes murmur3
- From
Derrick Stolee <derrickstolee@github.com>
- Date
- Jun 20, 2023, 13:39 UTC
- Message-ID
- <4c5cadf2-bd70-c56e-3935-fb24b604f411@github.com>
- In-Reply-To
- <5c65bf8a22d657c58b1109928b4298e95ccb56bf.1686677910.git.jonathantanmy@google.com>
On 6/13/2023 1:39 PM, Jonathan Tan wrote:
Show 7 quoted lines
> commitGraph.changedPathsVersion:: > Specifies the version of the changed-path Bloom filters that Git will read and > - write. May be 0 or 1. Any changed-path Bloom filters on disk that do not > + write. May be 0, 1, or 2. Any changed-path Bloom filters on disk that do not > match the version set in this config variable will be ignored. > + > Defaults to 1.
Is this a good place to document the planned modification of this default in future versions of Git?
Show 14 quoted lines
> +static uint32_t murmur3_seeded_v1(uint32_t seed, const char *data, size_t len)
> {
> const uint32_t c1 = 0xcc9e2d51;
> const uint32_t c2 = 0x1b873593;
> @@ -130,8 +187,10 @@ void fill_bloom_key(const char *data,
> int i;
> const uint32_t seed0 = 0x293ae76f;
> const uint32_t seed1 = 0x7e646e2c;
> - const uint32_t hash0 = murmur3_seeded(seed0, data, len);
> - const uint32_t hash1 = murmur3_seeded(seed1, data, len);
> + const uint32_t hash0 = (settings->hash_version == 2
> + ? murmur3_seeded_v2 : murmur3_seeded_v1)(seed0, data, len);
> + const uint32_t hash1 = (settings->hash_version == 2
> + ? murmur3_seeded_v2 : murmur3_seeded_v1)(seed1, data, len);This is the critical step. I've not seen this ternary trick within a function name choice, but it makes for a compact check. Nice.
However, I think the 'settings->hash_version' is the wrong place to look for the condition. We should be getting this value from the commit-graph we are reading. (More on this later.)
Show 10 quoted lines
> struct bloom_filter_settings {
> /*
> * The version of the hashing technique being used.
> - * We currently only support version = 1 which is
> + * The newest version is 2, which is
> * the seeded murmur3 hashing technique implemented
> - * in bloom.c.
> + * in bloom.c. Bloom filters of version 1 were created
> + * with prior versions of Git, which had a bug in the
> + * implementation of the hash function....
Show 18 quoted lines
> +struct graph_read_bloom_data_data {
> + struct commit_graph *g;
> + int commit_graph_changed_paths_version;
> +};
> +
> static int graph_read_bloom_data(const unsigned char *chunk_start,
> size_t chunk_size, void *data)
> {
> - struct commit_graph *g = data;
> + struct graph_read_bloom_data_data *d = data;
> + struct commit_graph *g = d->g;
> uint32_t hash_version;
> g->chunk_bloom_data = chunk_start;
> hash_version = get_be32(chunk_start);
>
> - if (hash_version != 1)
> + if (hash_version != d->commit_graph_changed_paths_version)
> return 0;This makes it appear like we cannot read a commit-graph that has a Bloom filter version that doesn't match the configured version.
This seems incorrect. If we want to configure to _write_ v2, we should still be able to _read_ v1 concurrently until those v2 filters are written.
This check should be:
if (hash_version <= 0 || hash_version > 2) return 0;
and then
g->filter_hash_version = hash_version;
to store this hash version somewhere in the graph. This way, we can read any commit-graph file and will not suffer performance problems in the time between setting the config value and writing the new commit-graph file.
> - if (s->commit_graph_changed_paths_version == 1) {
> + if (s->commit_graph_changed_paths_version == 1
> + || s->commit_graph_changed_paths_version == 2) {Perhaps this could just be
if (s->commit_graph_changed_paths_version) {to say "not zero means we still read the filters". Though, since this config _should_ mean "which version do we _write_?" it might be good to go back on the "unifying the two config options".
Show 10 quoted lines
> + struct graph_read_bloom_data_data data = {
> + .g = graph,
> + .commit_graph_changed_paths_version = s->commit_graph_changed_paths_version
> + };
> pair_chunk(cf, GRAPH_CHUNKID_BLOOMINDEXES,
> &graph->chunk_bloom_indexes);
> read_chunk(cf, GRAPH_CHUNKID_BLOOMDATA,
> - graph_read_bloom_data, graph);
> + graph_read_bloom_data, &data);
> }Much of this block will not be necessary as we don't need to send the repo settings into the read_chunk() method.
Show 5 quoted lines
> + if (r->settings.commit_graph_changed_paths_version < 0
> + || r->settings.commit_graph_changed_paths_version > 2) {
> + warning(_("attempting to write a commit-graph, but 'commitgraph.changedPathsVersion' (%d) is not supported"),
> + r->settings.commit_graph_changed_paths_version);
> + return 0;I see the "< 0" means we aren't considering the case of disabling writes with the zero value. We should exit early if we see a zero here, for extra safety.
> + } > + bloom_settings.hash_version = r->settings.commit_graph_changed_paths_version == 2 > + ? 2 : 1;
Once we've checked that this value is not zero, we can do this:
bloom_settings.hash_version = r->settings.commit_graph_changed_paths_version;
Show 6 quoted lines
> /* We have changed-paths already. Keep them in the next graph */
> - if (g && g->chunk_bloom_data) {
> + if (g && g->bloom_filter_settings) {
> ctx->changed_paths = 1;
> ctx->bloom_settings = g->bloom_filter_settings;
> }Show 11 quoted lines
> --- a/t/t4216-log-bloom.sh > +++ b/t/t4216-log-bloom.sh > @@ -450,4 +450,35 @@ test_expect_success 'version 1 changed-path used when version 1 requested' ' > test_bloom_filters_used "-- $CENT") > ' > > +test_expect_success 'version 1 changed-path not used when version 2 requested' ' > + (cd highbit1 && > + git config --add commitgraph.changedPathsVersion 2 && > + test_bloom_filters_not_used "-- $CENT") > +'
I think this test is the wrong behavior. We should be able to use version 1 when version 2 is requested.
Instead, start with version 1, and _upgrade_ to version 2 and then check which version exists in the file.
We should only _not_ use the filters if the version is 0 (or commitGraph.readChangedPaths=false). I think this might be a good enough reason to
Show 25 quoted lines
> + > +test_expect_success 'set up repo with high bit path, version 2 changed-path' ' > + git init highbit2 && > + git -C highbit2 config --add commitgraph.changedPathsVersion 2 && > + test_commit -C highbit2 c2 "$CENT" && > + git -C highbit2 commit-graph write --reachable --changed-paths > +' > + > +test_expect_success 'check value of version 2 changed-path' ' > + (cd highbit2 && > + printf "c01f" >expect && > + get_first_changed_path_filter >actual && > + test_cmp expect actual) > +' > + > +test_expect_success 'version 2 changed-path used when version 2 requested' ' > + (cd highbit2 && > + test_bloom_filters_used "-- $CENT") > +' > + > +test_expect_success 'version 2 changed-path not used when version 1 requested' ' > + (cd highbit2 && > + git config --add commitgraph.changedPathsVersion 1 && > + test_bloom_filters_not_used "-- $CENT") > +'
Again, this is also the wrong situation.
Thanks, -Stolee