Re: Effect of changing the value for PARALLEL_TUPLE_QUEUE_SIZE

On Wed, Sep 6, 2017 at 4:14 PM, Rafia Sabih
<rafia(dot)sabih(at)enterprisedb(dot)com> wrote:

> I worked on this idea of using local queue as a temporary buffer to
> write the tuples when master is busy and shared queue is full, and it
> gives quite some improvement in the query performance.
>

I have done some initial review of this patch and I have some comments.

/* this is actual size for this tuple which will be written in queue */
+ tuple_size = MAXALIGN(sizeof(Size)) + MAXALIGN(iov.len);
+
+ /* create and attach a local queue, if it is not yet created */
+ if (mqh->mqh_local_queue == NULL)
+ mqh = local_mq_attach(mqh);

I think we can create the local queue when first time we need it. So
basically you
can move this code inside else part where we first identify that there is no
space in the shared queue.

------
+ /* write in local queue if there is enough space*/
+ if (local_space > tuple_size)

I think the condition should be if (local_space >= tuple_size)

------
+ while(shm_space <= 0)
+ {
+ if (shm_mq_is_detached(mqh->mqh_queue))
+ return SHM_MQ_DETACHED;
+
+ shm_space = space_in_shm(mqh->mqh_queue);
+ }

Instead of waiting in CPU intensive while loop we should wait on some latch, why
can't we use wait latch of the shared queue and whenever some space
free, the latch will
be set then we can recheck the space and if it's enough we can write
to share queue
otherwise wait on the latch again.

Check other similar occurrences.
---------

+ if (read_local_queue(lq, true) && shm_space > 0)
+ copy_local_to_shared(lq, mqh, false);
+

Instead of adding shm_space > 0 in check it can be Assert or nothing
because above loop will
only break if shm_space > 0.
----

+ /*
+ * create a local queue, the size of this queue should be way higher
+ * than PARALLEL_TUPLE_QUEUE_SIZE
+ */
+ char *mq;
+ Size len;
+
+ len = 6553600;

Create some macro which is multiple of PARALLEL_TUPLE_QUEUE_SIZE,

-------

+ /* this local queue is not required anymore, hence free the space. */
+ pfree(mqh->mqh_local_queue);
+ return;
+}

I think you can remove the return at the end of the void function.
-----

In empty_queue(shm_mq_handle *mqh) function I see that only last exit path frees
the local queue but not all even though local queue is created.
----

Other cosmetic issues.
-----------------------------
+/* check the space availability in local queue */
+static uint64
+space_in_local(local_mq *lq, Size tuple_size)
+{
+ uint64 read, written, used, available, ringsize, writer_offset, reader_offset;
+
+ ringsize = lq->mq_ring_size;
+ read = lq->mq_bytes_read;
+ written = lq->mq_bytes_written;
+ used = written - read;
+ available = ringsize - used;
+
Alignment is not correct, I think you can run pgindent once.
----

+ /* check is there is required space in shared queue */
statement need refactoring. "check if there is required space in shared queue" ?

-----

+ /* write in local queue if there is enough space*/
space missing before comment end.

-----

+
+/* Routines required for local queue */
+
+/*
+ * Initialize a new local message queue, this is kept quite similar
to shm_mq_create.
+ */

Header comments formatting is not in sync with other functions.

-----

+}
+/* routine to create and attach local_mq to the shm_mq_handle */

one blank line between two functions.

-----

+ bool detached;
+
+ detached = false;

a better way is bool detached = false;

-----

Compilation warning
--------------------
shm_mq.c: In function ‘write_in_local_queue’:
shm_mq.c:1489:32: warning: variable ‘tuple_size’ set but not used
[-Wunused-but-set-variable]
uint64 bytes_written, nbytes, tuple_size;

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com