Re: Effect of changing the value for PARALLEL_TUPLE_QUEUE_SIZE

From: Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>
To: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
Cc: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, PostgreSQL Developers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Effect of changing the value for PARALLEL_TUPLE_QUEUE_SIZE
Date: 2017-09-25 09:13:00
Message-ID: CAOGQiiMOWJwfaegpERkvv3t6tY2CBdnhWHWi1iCfuMsCC98a4g@mail.gmail.com
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On Thu, Sep 21, 2017 at 10:34 PM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> On Thu, Sep 21, 2017 at 4:50 PM, Rafia Sabih
> <rafia(dot)sabih(at)enterprisedb(dot)com> wrote:
>> On Sun, Sep 17, 2017 at 9:10 PM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>>> On Wed, Sep 6, 2017 at 4:14 PM, Rafia Sabih
>>> <rafia(dot)sabih(at)enterprisedb(dot)com> wrote:
>>>
>>
>> Please find the attached file for the revised version.
>
> Thanks for the updated patch, I have some more comments.
>
Again, thanks for the review. Please find the attached file for updated patch.

> +static shm_mq_handle *local_mq_attach(shm_mq_handle *mqh);
> +static uint64 space_in_local(local_mq * lq, Size tuple_size);
> +static bool read_local_queue(local_mq * lq, bool shm_mq_full);
>
> local_mq * lq -> local_mq *lq
> same for other places as well.
>
> ---
>
Done. This is something pgindent does, anyhow corrected it.

> +static uint64 space_in_shm(shm_mq *mq);
> +
> +static uint64 space_in_local(local_mq * lq, Size tuple_size);
>
> we better use Size here instead if uint64
>
> ---
Done
>
> + available = ringsize - used;
> +
> + ringsize = lq->mq_ring_size;
> + writer_offset = lq->mq_bytes_written % ringsize;
> + reader_offset = lq->mq_bytes_read % ringsize;
> +
> + if (writer_offset + tuple_size < ringsize && reader_offset < writer_offset)
> + available = (ringsize - writer_offset);
>
> even though there is space in queue but tuple need rotation then we
> are not allowing it to
> write into the local queue. If there is some strong reason behind
> that, please add comments
> to explain the same.
> ---
>
Corrected, it will just return available space now.

> + if (shm_mq_full || (written - read) >= .05 * lq->mq_ring_size)
> + return true;
> +
> + else
> + return true;
> +}
>
> Seems like you want to return 'false' in the else case.
>
> ----
Yes and done.
>
> + read_offset = lq->mq_bytes_read % lq->mq_ring_size;
> + available = space_in_shm(mqh->mqh_queue);
> +
> + /* always read data in the aligned form */
> + to_read = MAXALIGN_DOWN(Min(used, available));
> +
> + /*
> + * if the amount of data to be send from local queue involves wrapping of
> + * local queue, then send only the data till the end of queue right now
> + * and rest later.
> + */
> + if (lq->mq_bytes_read % lq->mq_ring_size + to_read > lq->mq_ring_size)
>
> You can directly use "read_offset" instead of recalculating
> lq->mq_bytes_read % lq->mq_ring_size.
>
> ----
Done.
> + do
> + {
> + if (shm_mq_is_detached(mqh->mqh_queue))
> + return SHM_MQ_DETACHED;
> +
> + shm_space = space_in_shm(mqh->mqh_queue);
> +
> + /*
> + * cannot send data to shared queue, unless there is required
> + * space, so wait till we get some space, since we cannot
> + * write anymore in local queue as of now
> + */
> + WaitLatch(MyLatch, WL_LATCH_SET, 0, WAIT_EVENT_MQ_SEND);
> +
> + /* Reset the latch so we don't spin. */
> + ResetLatch(MyLatch);
> +
> + /* An interrupt may have occurred while we were waiting. */
> + CHECK_FOR_INTERRUPTS();
> +
> + shm_space = space_in_shm(mqh->mqh_queue);
> +
> + if (read_local_queue(lq, true) && shm_space > 0)
> + copy_local_to_shared(lq, mqh, false);
> +
> + local_space = space_in_local(lq, tuple_size);
> +
> + } while (local_space <= tuple_size);
> +
>
> 1. Just after getting the shm_space, you are calling WaitLatch,
> without even checking whether
> that space is sufficient to send the tuple.
> 2. Every time after latch is set (maybe some space freed in the shared
> queue) you are calling
> copy_local_to_shared to send as much data as possible from local to
> shared queue, but that doesn't
> even guarantee that we will have sufficient space in the local queue
> to accommodate the current tuple.
>
> I think calling copy_local_to_shared multiple time (which will
> internally acquire mutex), after latch
> is set you can check the shared queue space, don't attempt
> copy_local_to_shared unless
> shm_space >=tuple_size
>
Done.
> --
> Regards,
> Dilip Kumar
> EnterpriseDB: http://www.enterprisedb.com

--
Regards,
Rafia Sabih
EnterpriseDB: http://www.enterprisedb.com/

Attachment Content-Type Size
faster_gather_v3.patch application/octet-stream 14.5 KB

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