After couple CPU profiling sessions with perf, it was detected that the
function pgstat_fetch_stat_numbackends() is very expensive, reading the
implementation on PostgreSQL's backend_status.c just confirmed that.
We use that function on pg_stat_monitor to retrieve the application name
and IP address of the client, we now cache the results in order to avoid
calling it for every query being processed.
If pgsm_overflow_target is ON (default, 1) and the query buffer
overflows, we now dump the buffer and keep track of how many times
pg_stat_monitor changed bucket since that.
If an overflow happen again before pg_stat_monitor cycle through
pgsm_max_buckets buckets (default 10), then we don't dump the buffer
again, but instead report an error, this ensures that only one dump file
of size pgsm_query_shared_buffer will be in disk at any time, avoiding
slowing down queries to the pg_stat_monitor view.
As soon as pg_stat_monitor cycles through all buckets, we remove the
dump file and reset the counter (pgss->n_bucket_cycles).
pgss_ExecutorEnd: Avoid unnecessary memset(plan_info, 0, ...).
We only use this object if the condition below is true, in which case we
already initialize all the fields in the object, also we now store the
plan string length (plan_info.plan_len) to avoid calling strlen on it
again later:
if (queryDesc->operation == CMD_SELECT && PGSM_QUERY_PLAN) {
... here we initialize plan_info
If the condition is false, then we pass a NULL PlanInfo* to the
pgss_store to avoid more unnecessary processing.
pgss_planner_hook: Similar, avoid memset(plan_info, 0, ...) this object
is not used here, so we pass NULL to pgss_store.
pg_get_application_name: Remove call to strlen, snprintf already give us
the calculated string length, so we just return it.
pg_get_client_addr: Cache localhost, avoid calling
ntohl(inet_addr("127.0.0.1")) all the time.
pgss_update_entry: Make use of PlanInfo->plan_len, avoiding a call to
strlen again.
intarray_get_datum: Init the string by setting the first byte to '\0'.
The memory area reserved for query text (pgsm_query_shared_buffer) was
divided evenly for each bucket, this allowed to have the same query,
e.g. "SELECT 1", duplicated in different buckets, thus wasting space.
This commit fix the query text duplication by adding a new hash table
whose only purpose is to verify if a given query is already added to the
buffer (by using the queryID).
This allows different buckets that share the same query to point to a
unique entry in the query buffer (pgss_qbuf).
When pg_stat_monitor moves to a new bucket id, by avoiding adding a
query that already exists in the buffer it can also save some CPU time.
Before PostgreSQL 14, HASH_STRINGS flag was not available when creating
a hash table with ShmemInitHash().
Use HASH_BLOBS for previous PostgreSQL versions.
The pg_stat_monitor_errors view was created in order to help inspecting
internal errors that may occur in pg_stat_monitor module itself, it
contains the error message, its severity, the last time the error occurred
and the number of times that any given error ocurred.
Implementation details:
- A new lwlock was added to the pgssSharedState structure in order to
control access to the errors hash table.
- Increased RequestNamedLWLockTranche() no. of locks requested to 2.
- The function GetNamedLWLockTranche() returns an array of locks, we
use the first lock for controlling access to the usual buckets hash
table, and the second one to control access to the errors hash table.
- During module initialization (_PG_init) we increased the amount of
shared memory space requested to include space to the new errors hash
table: RequestAddinShmemSpace(... + pgsm_errors_size())
- The implementation in pgsm_errors.c simple uses a hash table to track
error messages, the message is also used as the key.
There were couple issues to handle, the main one was that our log hook
(pgsm_emit_log_hook) was being called after the shared memory hook
completed (pgss_shmem_startup) but before PostgreSQL boostraping code
finished, thus triggering the following assertion during a call to
LWLockAcquire():
Assert(!(proc == NULL && IsUnderPostmaster));
proc is a pointer to MyProc, a PostgreSQL's shared global variable that
was not yet initalized by PostgreSQL.
We must also check for a NULL pointer return in pg_get_backend_status()
the pgstat_fetch_stat_local_beentry() function may return a NULL pointer
during initialization, in which case we use "127.0.0.1" for the client
address, and "postmaster" for application name.
If both modules are loaded then pg_stat_monitor detects that and avoid
calling standard_ProcessUtility() in ProcessUtility_hook hook, as
calling it twice is an error and triggers an assertion on PostgreSQL.
On PostgreSQL 13, pg_stat_monitor must be loaded after
pg_stat_statements, as pg_stat_statements doesn't do such verifications,
it end calling standard_ProcessUtility() and other functions even if
another module is registered, that is an error.
They fixed this problem with pg_stat_statements in PostgreSQL 14 and onward.
The loop that resets the query buffers was incorrecly using MAX_BUCKETS
to indicate the number of buckets to clear, which defaults to 10. If a
user lowers this value the loop would access a pointer beyond the number
of query buffers allocated.
Fix the problem by using the correct PGSM_MAX_BUCKETS GUC as the limit
to the loop.
