ovsdb: Prepare snapshot JSON in a separate thread.

Conversion of the database data into JSON object, serialization
and destruction of that object are the most heavy operations
during the database compaction.  If these operations are moved
to a separate thread, the main thread can continue processing
database requests in the meantime.

With this change, the compaction is split in 3 phases:

1. Initialization:
   - Create a copy of the database.
   - Remember current database index.
   - Start a separate thread to convert a copy of the database
     into serialized JSON object.

2. Wait:
   - Continue normal operation until compaction thread is done.
   - Meanwhile, compaction thread:
     * Convert database copy to JSON.
     * Serialize resulted JSON.
     * Destroy original JSON object.

3. Finish:
   - Destroy the database copy.
   - Take the snapshot created by the thread.
   - Write on disk.

The key for this schema to be fast is the ability to create
a shallow copy of the database.  This doesn't take too much
time allowing the thread to do most of work.

Database copy is created and destroyed only by the main thread,
so there is no need for synchronization.

Such solution allows to reduce the time main thread is blocked
by compaction by 80-90%.  For example, in ovn-heater tests
with 120 node density-heavy scenario, where compaction normally
takes 5-6 seconds at the end of a test, measured compaction
times was all below 1 second with the change applied.  Also,
note that these measured times are the sum of phases 1 and 3,
so actual poll intervals are about half a second in this case.

Only implemented for raft storage for now.  The implementation
for standalone databases can be added later by using a file
offset as a database index and copying newly added changes
from the old file to a new one during ovsdb_log_replace().

Reported-at: https://bugzilla.redhat.com/2069108
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 6001aeb1d..c2da6bac1 100644
--- a/NEWS
+++ b/NEWS
@@ -24,6 +24,8 @@ Post-v2.17.0
      * Returning unused memory to the OS after the database compaction is now
        enabled by default.  Use 'ovsdb-server/memory-trim-on-compaction off'
        unixctl command to disable.
+     * Most of the work for the automatic database compaction in clustered
+       mode has been moved to a separate thread to avoid blocking the process.
    - OVSDB-IDL:
      * New monitor mode flag, OVSDB_IDL_WRITE_CHANGED_ONLY, allowing
        applications to relax atomicity requirements when dealing with