Re: Parallel bitmap heap scan

From: Dilip Kumar <dilipbalaut(at)gmail(dot)com>
To: Haribabu Kommi <kommi(dot)haribabu(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Rafia Sabih <rafia(dot)sabih(at)enterprisedb(dot)com>, tushar <tushar(dot)ahuja(at)enterprisedb(dot)com>, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Parallel bitmap heap scan
Date: 2017-01-31 11:05:57
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On Tue, Jan 31, 2017 at 7:47 AM, Haribabu Kommi
<kommi(dot)haribabu(at)gmail(dot)com> wrote:
> Thanks for the update. I have some comments
Thanks for the review.

> 0002-hash-support-alloc-free-v14.patch:
> + if (tb->alloc)
> + {
> The memory for tb->alloc is allocated always, is the if check still
> required?

In parallel case, only first worker will call SH_CREATE, other worker
will only do palloc for pagetable and copy the reference from main
worker, so they will not have allocator.

> 0003-parallel-bitmap-heap-scan-v14.patch:
> + * and set parallel flag in lower level bitmap index scan. Later
> + * bitmap index node will use this flag to indicate tidbitmap that
> + * it needs to create an shared page table.
> + */
> I feel better to mention, where this flag is used, so that it will be more
> clear.

> + * Mark tidbitmap as shared and also store DSA area in it.
> + * marking tidbitmap as shared is indication that this tidbitmap
> + * should be created in shared memory. DSA area will be used for
> The flag of shared is set in tidbitmap structure itself, but the
> comment is mentioned that tidbitmpa should be created in shared memory.
> I think that is the page table that needs to be created in shared memory.
> Providing some more information here will be helpful.

> - node->tbmres = tbmres = tbm_iterate(tbmiterator);
> + node->tbmres = tbmres = pbms_parallel_iterate(tbmiterator,
> + pbminfo ? &pbminfo->tbmiterator : NULL);
> Instead Passing both normal and paralle iterators to the functions
> and checking inside again for NULL, How about just adding check
> in the caller itself? Or if you prefer the current method, then try to
> explain the details of input in the function header and more description.
Done as you said.

> + /* Increase prefetch target if it's not yet at the max. */
> + if (node->prefetch_target < node->prefetch_maximum)
> I didn't evaluate all scenarios, but the above code may have a problem,
> In case of parallel mode the the prefetch_target is fetched from node
> and not from the pbminfo. Later it gets from the pminfo and update that.
> May be this code needs to rewrite.
Fixed it.

> + TBMIterator *iterator = node->prefetch_iterator;
> Why another variable? Why can't we use the prefetch_iterator itself?
> Currently node->tbmiterator and node->prefetch_iterator are initialized
> irrespective of whether is it a parallel one or not. But while using, there
> is a check to use the parallel one in case of parallel. if it is the case,
> why can't we avoid the initialization itself?

> + else
> + needWait = false;
> By default needWait is false. Just set that to true only for the

Actually inside the while loop, suppose first we set needWait = true,
if PBM_INPROGRESS and got for ConditionalSleep, After it come out of
sleep, we need to check that PBM_FINISHED is set or we need to sleep
again, So in such case we need to reset it to needWait=false. However
this can be done by directly returning if it's PBM_FINISHED. But I
want to keep below the logic common.
+ SpinLockRelease(&pbminfo->state_mutex);
+ /* If we are leader or leader has already created a TIDBITMAP */
+ if (leader || !needWait)
+ break;

> + * so that during free we can directly get the dsa_pointe and free it.
> dsa_pointe -> dsa_pointer

> +typedef struct
> +{
> + TIDBitmap *tbm; /* TIDBitmap we're iterating over */
> + int spageptr; /* next spages index */
> + int schunkptr; /* next schunks index */
> + int schunkbit; /* next bit to check in current schunk */
> + TBMIterateResult output; /* MUST BE LAST (because variable-size) */
> +} TBMIterator;
> I didn't find the need of moving this structure. Can you point me where it
> is used.

Because pbms_parallel_iterate need to access this structure so I
moved it to tidbitmap.h

Dilip Kumar

Attachment Content-Type Size
0003-parallel-bitmap-heap-scan-v15.patch application/octet-stream 44.9 KB

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