On Wed, Apr 20, 2022 at 12:31 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. > > 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]>
Thanks Dumitru for the revision. I only have two minor comments below. Acked-by: Han Zhou <[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. s/chave/change > + * > + * 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. In the original form of this comment, we need to mention "OVSDB_IDL_MONITOR | OVSDB_IDL_ALERT | OVSDB_IDL_TRACK | OVSDB_IDL_WRITE_CHANGED_ONLY" as well, but I think it is too tedious. Maybe we can change the section from "possible mode combinations" to "typical mode combinations". > */ > #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
