On Mon, Apr 25, 2022 at 2:28 AM Dumitru Ceara <[email protected]> wrote: > > On 4/22/22 20:38, Terry Wilson wrote: > > On Wed, Apr 20, 2022 at 2:31 PM Dumitru Ceara <[email protected]> wrote: > >> > >> At a first glance, change tracking should never be allowed for > >> write-only columns. However, some clients (e.g., ovn-northd) that are > >> mostly exclusive writers of a database, use change tracking to avoid > >> duplicating the IDL row records into a local cache when implementing > >> incremental processing. > >> > >> The default behavior of the IDL is to automatically turn a write-only > >> column into a read-write column whenever the client enables change > >> tracking for that column. > >> > >> For the afore mentioned clients, this becomes a performance issue. > >> Commit 1cc618c32524 ("ovsdb-idl: Fix atomicity of writes that don't > >> change a column's value.") explains why writes that don't change a > >> column's value cannot be optimized out early if the column is > >> read/write. > >> > >> Furthermore, if there is at least one record in any table that > >> changed during a transaction, then *all* records that have been > >> written are added to the transaction, even if their values didn't > >> change. If there are many such rows (e.g., like in ovn-northd's > >> case) this incurs a significant overhead because: > >> a. the client has to build this large transaction > >> b. the transaction has to be sent over the network > >> c. the server needs to parse this (mostly) no-op update > >> > >> We now introduce new IDL APIs allowing users to set a new monitoring > >> mode flag, OVSDB_IDL_WRITE_CHANGED_ONLY, to indicate to the IDL that the > >> atomicity constraints may be relaxed and written columns that don't > >> change value can be skipped from the current transaction. > > > > Is this something that we should consider adding at some point to the > > Python IDL code as well? I know in the past feature support has > > diverged. Maybe at some point we should create a python/TODO file that > > lists features in the C IDL that still need to be implemented if they > > aren't immediately added with C IDL changes? > > You're right, we should [robably consider adding this as a new mode to > the Python IDL too. I can do that as a follow up if needed. However, > until now, the only user of this new mode is ovn-northd and it will be > using it in conjunction with change tracking (which is another missing > Python IDL feature). > > Han, Ilya, what do you think? > > Nevertheless, a TODO list would probably be good to have indeed. > I agree that a TODO would be good for the python IDL. Since there are already feature gaps, I think it may be a separate patch to add TODOs for all the gaps.
Thanks, Han > > > >> We benchmarked ovn-northd performance when using this new mode > >> against NB and SB databases taken from ovn-kubernetes scale tests. > >> We noticed that when a minor change is performed to the Northbound > >> database (e.g., NB_Global.nb_cfg is incremented) the time it takes to > >> build the Southbound transaction becomes negligible (vs ~1.5 seconds > >> before this change). > >> > >> End-to-end ovn-kubernetes scale tests on 120-node clusters also show > >> significant reduction of latency to bring up pods; both average and P99 > >> latency decreased by ~30%. > >> > >> Signed-off-by: Dumitru Ceara <[email protected]> > >> --- > >> V3: > >> - Addressed Han's comments: > >> - Add a new IDL monitor mode flag, OVSDB_IDL_WRITE_CHANGED_ONLY. > >> - Rephrased commit log. > >> - Changed commit title to reflect the new approach. > >> - Old patch (v2) was: > >> https://patchwork.ozlabs.org/project/openvswitch/patch/[email protected]/ > >> V2: > >> - Addressed Numan's comments: > >> - Added APIs to allow per column configuration of modes. > >> - Fixed unit tests to actually enable change tracking for write-only > >> columns. > >> - Fixed ovsdb_idl_row_change() to correctly update row/table/idl change > >> seqnos if change tracking is enabled (even for write-only rows). > >> > >> Note: The OVN counter part change is: > >> https://github.com/dceara/ovn/commit/5afba6f3aa51f508df41d8f0b7b9d739b00b1ee2 > >> --- > >> NEWS | 4 ++++ > >> lib/ovsdb-idl.c | 57 +++++++++++++++++++++++++++++++++++++++++----- > >> lib/ovsdb-idl.h | 14 +++++++++++- > >> tests/ovsdb-idl.at | 31 ++++++++++++++++++++++++- > >> tests/test-ovsdb.c | 18 +++++++++++---- > >> 5 files changed, 111 insertions(+), 13 deletions(-) > >> > >> diff --git a/NEWS b/NEWS > >> index 5074b97aa51c..60932791bfcf 100644 > >> --- a/NEWS > >> +++ b/NEWS > >> @@ -14,6 +14,10 @@ Post-v2.17.0 > >> - OVSDB: > >> * 'relay' service model now supports transaction history, i.e. honors the > >> 'last-txn-id' field in 'monitor_cond_since' requests from clients. > >> + - OVSDB-IDL: > >> + * New monitor mode flag, OVSDB_IDL_WRITE_CHANGED_ONLY, allowing > >> + applications to relax atomicity requirements when dealing with > >> + columns whose value has been rewritten (but not changed). > >> > >> > >> v2.17.0 - 17 Feb 2022 > >> diff --git a/lib/ovsdb-idl.c b/lib/ovsdb-idl.c > >> index 882ede75598d..e5bb6cca5274 100644 > >> --- a/lib/ovsdb-idl.c > >> +++ b/lib/ovsdb-idl.c > >> @@ -1396,6 +1396,42 @@ ovsdb_idl_track_clear(struct ovsdb_idl *idl) > >> { > >> ovsdb_idl_track_clear__(idl, false); > >> } > >> + > >> +/* Sets or clears (depending on 'enable') OVSDB_IDL_WRITE_CHANGED_ONLY > >> + * for 'column' in 'idl', ensuring that the column will be included in a > >> + * transaction only if its value has actually changed locally. Normally > >> + * read/write columns that are written to are always included in the > >> + * transaction but, in specific cases, when the application doesn't > >> + * require atomicity of writes across different columns the ones that > >> + * don't chave value may be skipped. > >> + * > >> + * This function should be called between ovsdb_idl_create() and > >> + * the first call to ovsdb_idl_run(). > >> + */ > >> +void > >> +ovsdb_idl_set_write_change_only(struct ovsdb_idl *idl, > >> + const struct ovsdb_idl_column *column, > >> + bool enable) > >> +{ > >> + if (enable) { > >> + *ovsdb_idl_get_mode(idl, column) |= OVSDB_IDL_WRITE_CHANGED_ONLY; > >> + } else { > >> + *ovsdb_idl_get_mode(idl, column) &= ~OVSDB_IDL_WRITE_CHANGED_ONLY; > >> + } > >> +} > >> + > >> +void > >> +ovsdb_idl_set_write_change_only_all(struct ovsdb_idl *idl, bool enable) > >> +{ > >> + for (size_t i = 0; i < idl->class_->n_tables; i++) { > >> + const struct ovsdb_idl_table_class *tc = &idl->class_->tables[i]; > >> + > >> + for (size_t j = 0; j < tc->n_columns; j++) { > >> + const struct ovsdb_idl_column *column = &tc->columns[j]; > >> + ovsdb_idl_set_write_change_only(idl, column, enable); > >> + } > >> + } > >> +} > >> > >> static void > >> log_parse_update_error(struct ovsdb_error *error) > >> @@ -3491,6 +3527,8 @@ ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_, > >> { > >> struct ovsdb_idl_row *row = CONST_CAST(struct ovsdb_idl_row *, row_); > >> const struct ovsdb_idl_table_class *class; > >> + unsigned char column_mode; > >> + bool optimize_rewritten; > >> size_t column_idx; > >> bool write_only; > >> > >> @@ -3501,12 +3539,15 @@ ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_, > >> > >> class = row->table->class_; > >> column_idx = column - class->columns; > >> - write_only = row->table->modes[column_idx] == OVSDB_IDL_MONITOR; > >> + column_mode = row->table->modes[column_idx]; > >> + write_only = column_mode == OVSDB_IDL_MONITOR; > >> + optimize_rewritten = > >> + write_only || (column_mode & OVSDB_IDL_WRITE_CHANGED_ONLY); > >> + > >> > >> ovs_assert(row->new_datum != NULL); > >> ovs_assert(column_idx < class->n_columns); > >> - ovs_assert(row->old_datum == NULL || > >> - row->table->modes[column_idx] & OVSDB_IDL_MONITOR); > >> + ovs_assert(row->old_datum == NULL || column_mode & OVSDB_IDL_MONITOR); > >> > >> if (row->table->idl->verify_write_only && !write_only) { > >> VLOG_ERR("Bug: Attempt to write to a read/write column (%s:%s) when" > >> @@ -3524,9 +3565,13 @@ ovsdb_idl_txn_write__(const struct ovsdb_idl_row *row_, > >> * different value in that column since we read it. (But if a whole > >> * transaction only does writes of existing values, without making any real > >> * changes, we will drop the whole transaction later in > >> - * ovsdb_idl_txn_commit().) */ > >> - if (write_only && ovsdb_datum_equals(ovsdb_idl_read(row, column), > >> - datum, &column->type)) { > >> + * ovsdb_idl_txn_commit().) > >> + * > >> + * The application may choose to bypass this restriction and always > >> + * optimize by setting OVSDB_IDL_WRITE_CHANGED_ONLY. > >> + */ > >> + if (optimize_rewritten && ovsdb_datum_equals(ovsdb_idl_read(row, column), > >> + datum, &column->type)) { > >> goto discard_datum; > >> } > >> > >> diff --git a/lib/ovsdb-idl.h b/lib/ovsdb-idl.h > >> index d00599616ef9..1716955ab61c 100644 > >> --- a/lib/ovsdb-idl.h > >> +++ b/lib/ovsdb-idl.h > >> @@ -181,10 +181,17 @@ bool ovsdb_idl_server_has_column(const struct ovsdb_idl *, > >> * - (OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT | OVSDB_IDL_TRACK), for a column > >> * that a client wants to track using the change tracking > >> * ovsdb_idl_track_get_*() functions. > >> + * > >> + * - (OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT | OVSDB_IDL_WRITE_CHANGED_ONLY) > >> + * is similar to (OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT) except that it > >> + * only adds a written column to a transaction if the column's value > >> + * has actually changed. > >> */ > >> #define OVSDB_IDL_MONITOR (1 << 0) /* Replicate this column? */ > >> #define OVSDB_IDL_ALERT (1 << 1) /* Alert client when column changes? */ > >> -#define OVSDB_IDL_TRACK (1 << 2) > >> +#define OVSDB_IDL_TRACK (1 << 2) /* Track column changes? */ > >> +#define OVSDB_IDL_WRITE_CHANGED_ONLY \ > >> + (1 << 3) /* Write back only changed columns? */ > >> > >> void ovsdb_idl_add_column(struct ovsdb_idl *, const struct ovsdb_idl_column *); > >> void ovsdb_idl_add_table(struct ovsdb_idl *, > >> @@ -233,6 +240,11 @@ bool ovsdb_idl_track_is_updated(const struct ovsdb_idl_row *row, > >> const struct ovsdb_idl_column *column); > >> void ovsdb_idl_track_clear(struct ovsdb_idl *); > >> > >> +void ovsdb_idl_set_write_change_only(struct ovsdb_idl *idl, > >> + const struct ovsdb_idl_column *column, > >> + bool enable); > >> +void ovsdb_idl_set_write_change_only_all(struct ovsdb_idl *idl, bool enable); > >> + > >> > >> /* Reading the database replica. */ > >> > >> diff --git a/tests/ovsdb-idl.at b/tests/ovsdb-idl.at > >> index 62e2b638320c..d1cfa59c5758 100644 > >> --- a/tests/ovsdb-idl.at > >> +++ b/tests/ovsdb-idl.at > >> @@ -87,6 +87,20 @@ m4_define([OVSDB_CHECK_IDL_C], > >> OVSDB_SERVER_SHUTDOWN > >> AT_CLEANUP]) > >> > >> +# same as OVSDB_CHECK_IDL but uses OVSDB_IDL_WRITE_CHANGED_ONLY. > >> +m4_define([OVSDB_CHECK_IDL_WRITE_CHANGED_ONLY_C], > >> + [AT_SETUP([$1 - write-changed-only - C]) > >> + AT_KEYWORDS([ovsdb server idl positive $5]) > >> + AT_CHECK([ovsdb_start_idltest]) > >> + m4_if([$2], [], [], > >> + [AT_CHECK([ovsdb-client transact unix:socket $2], [0], [ignore], [ignore])]) > >> + AT_CHECK([test-ovsdb '-vPATTERN:console:test-ovsdb|%c|%m' -vjsonrpc -t10 idl unix:socket $3], > >> + [0], [stdout], [ignore]) > >> + AT_CHECK([sort stdout | uuidfilt]m4_if([$6],,, [[| $6]]), > >> + [0], [$4]) > >> + OVSDB_SERVER_SHUTDOWN > >> + AT_CLEANUP]) > >> + > >> # same as OVSDB_CHECK_IDL but uses tcp. > >> m4_define([OVSDB_CHECK_IDL_TCP_C], > >> [AT_SETUP([$1 - C - tcp]) > >> @@ -257,6 +271,7 @@ m4_define([OVSDB_CHECK_IDL_SSL_PY], > >> > >> m4_define([OVSDB_CHECK_IDL], > >> [OVSDB_CHECK_IDL_C($@) > >> + OVSDB_CHECK_IDL_WRITE_CHANGED_ONLY_C($@) > >> OVSDB_CHECK_IDL_TCP_C($@) > >> OVSDB_CHECK_IDL_TCP6_C($@) > >> OVSDB_CHECK_IDL_PY($@) > >> @@ -1166,8 +1181,22 @@ m4_define([OVSDB_CHECK_IDL_TRACK_C], > >> OVSDB_SERVER_SHUTDOWN > >> AT_CLEANUP]) > >> > >> +m4_define([OVSDB_CHECK_IDL_TRACK_WRITE_CHANGED_ONLY_C], > >> + [AT_SETUP([$1 - write-changed-only - C]) > >> + AT_KEYWORDS([ovsdb server idl tracking positive $5]) > >> + AT_CHECK([ovsdb_start_idltest]) > >> + m4_if([$2], [], [], > >> + [AT_CHECK([ovsdb-client transact unix:socket $2], [0], [ignore], [ignore])]) > >> + AT_CHECK([test-ovsdb '-vPATTERN:console:test-ovsdb|%c|%m' -vjsonrpc -t10 -c -w idl unix:socket $3], > >> + [0], [stdout], [ignore]) > >> + AT_CHECK([sort stdout | uuidfilt]m4_if([$6],,, [[| $6]]), > >> + [0], [$4]) > >> + OVSDB_SERVER_SHUTDOWN > >> + AT_CLEANUP]) > >> + > >> m4_define([OVSDB_CHECK_IDL_TRACK], > >> - [OVSDB_CHECK_IDL_TRACK_C($@)]) > >> + [OVSDB_CHECK_IDL_TRACK_C($@) > >> + OVSDB_CHECK_IDL_TRACK_WRITE_CHANGED_ONLY_C($@)]) > >> > >> OVSDB_CHECK_IDL_TRACK([track, simple idl, initially populated], > >> [['["idltest", > >> diff --git a/tests/test-ovsdb.c b/tests/test-ovsdb.c > >> index ca4e87b8115c..808b15355743 100644 > >> --- a/tests/test-ovsdb.c > >> +++ b/tests/test-ovsdb.c > >> @@ -56,6 +56,7 @@ > >> VLOG_DEFINE_THIS_MODULE(test_ovsdb); > >> > >> struct test_ovsdb_pvt_context { > >> + bool write_changed_only; > >> bool track; > >> }; > >> > >> @@ -91,6 +92,7 @@ parse_options(int argc, char *argv[], struct test_ovsdb_pvt_context *pvt) > >> {"timeout", required_argument, NULL, 't'}, > >> {"verbose", optional_argument, NULL, 'v'}, > >> {"change-track", optional_argument, NULL, 'c'}, > >> + {"write-changed-only", optional_argument, NULL, 'w'}, > >> {"magic", required_argument, NULL, OPT_MAGIC}, > >> {"no-rename-open-files", no_argument, NULL, OPT_NO_RENAME_OPEN_FILES}, > >> {"help", no_argument, NULL, 'h'}, > >> @@ -125,6 +127,10 @@ parse_options(int argc, char *argv[], struct test_ovsdb_pvt_context *pvt) > >> pvt->track = true; > >> break; > >> > >> + case 'w': > >> + pvt->write_changed_only = true; > >> + break; > >> + > >> case OPT_MAGIC: > >> magic = optarg; > >> break; > >> @@ -2610,6 +2616,7 @@ update_conditions(struct ovsdb_idl *idl, char *commands) > >> static void > >> do_idl(struct ovs_cmdl_context *ctx) > >> { > >> + struct test_ovsdb_pvt_context *pvt = ctx->pvt; > >> struct jsonrpc *rpc; > >> struct ovsdb_idl *idl; > >> unsigned int seqno = 0; > >> @@ -2618,9 +2625,6 @@ do_idl(struct ovs_cmdl_context *ctx) > >> int step = 0; > >> int error; > >> int i; > >> - bool track; > >> - > >> - track = ((struct test_ovsdb_pvt_context *)(ctx->pvt))->track; > >> > >> idl = ovsdb_idl_create(ctx->argv[1], &idltest_idl_class, true, true); > >> ovsdb_idl_set_leader_only(idl, false); > >> @@ -2637,10 +2641,14 @@ do_idl(struct ovs_cmdl_context *ctx) > >> rpc = NULL; > >> } > >> > >> - if (track) { > >> + if (pvt->track) { > >> ovsdb_idl_track_add_all(idl); > >> } > >> > >> + if (pvt->write_changed_only) { > >> + ovsdb_idl_set_write_change_only_all(idl, true); > >> + } > >> + > >> setvbuf(stdout, NULL, _IONBF, 0); > >> > >> symtab = ovsdb_symbol_table_create(); > >> @@ -2683,7 +2691,7 @@ do_idl(struct ovs_cmdl_context *ctx) > >> } > >> > >> /* Print update. */ > >> - if (track) { > >> + if (pvt->track) { > >> print_idl_track(idl, step++, terse); > >> ovsdb_idl_track_clear(idl); > >> } else { > >> -- > >> 2.27.0 > >> > >> _______________________________________________ > >> dev mailing list > >> [email protected] > >> https://mail.openvswitch.org/mailman/listinfo/ovs-dev > >> > > > _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
