Date: Wed, 2 Sep 2015 15:53:51 +0200
From: Andres Freund <andres@anarazel.de>
To: Julien Rouhaud <julien.rouhaud@dalibo.com>
Cc: Jeff Janes <jeff.janes@gmail.com>, Bruce Momjian <bruce@momjian.us>,
 Merlin Moncure <mmoncure@gmail.com>,
 "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: Allow a per-tablespace effective_io_concurrency setting
Message-ID: <20150902135351.GA5286@alap3.anarazel.de>
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 <20141209204337.GC24488@momjian.us>
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Hi,

On 2015-07-18 12:17:39 +0200, Julien Rouhaud wrote:
> I didn't know that the thread must exists on -hackers to be able to add
> a commitfest entry, so I transfer the thread here.

Please, in the future, also update the title of the thread to something
fitting.

> @@ -539,6 +541,9 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
>  {
>  	BitmapHeapScanState *scanstate;
>  	Relation	currentRelation;
> +#ifdef USE_PREFETCH
> +	int new_io_concurrency;
> +#endif
>  
>  	/* check for unsupported flags */
>  	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
> @@ -598,6 +603,25 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
>  	 */
>  	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
>  
> +#ifdef USE_PREFETCH
> +	/* check if the effective_io_concurrency has been overloaded for the
> +	 * tablespace storing the relation and compute the target_prefetch_pages,
> +	 * or just get the current target_prefetch_pages
> +	 */
> +	new_io_concurrency = get_tablespace_io_concurrency(
> +			currentRelation->rd_rel->reltablespace);
> +
> +
> +	scanstate->target_prefetch_pages = target_prefetch_pages;
> +
> +	if (new_io_concurrency != effective_io_concurrency)
> +	{
> +		double prefetch_pages;
> +	   if (compute_io_concurrency(new_io_concurrency, &prefetch_pages))
> +			scanstate->target_prefetch_pages = rint(prefetch_pages);
> +	}
> +#endif

Maybe it's just me - but imo there should be as few USE_PREFETCH
dependant places in the code as possible. It'll just be 0 when not
supported, that's fine? Especially changing the size of externally
visible structs depending on a configure detected ifdef seems wrong to
me.

> +bool
> +compute_io_concurrency(int io_concurrency, double *target_prefetch_pages)
> +{
> +	double		new_prefetch_pages = 0.0;
> +	int			i;
> +
> +	/* make sure the io_concurrency value is correct, it may have been forced
> +	 * with a pg_tablespace UPDATE
> +	 */

Nitpick: Wrong comment style (/* stands on its own line).

> +	if (io_concurrency > MAX_IO_CONCURRENCY)
> +		io_concurrency = MAX_IO_CONCURRENCY;
> +
> +	/*----------
> +	 * The user-visible GUC parameter is the number of drives (spindles),
> +	 * which we need to translate to a number-of-pages-to-prefetch target.
> +	 * The target value is stashed in *extra and then assigned to the actual
> +	 * variable by assign_effective_io_concurrency.
> +	 *
> +	 * The expected number of prefetch pages needed to keep N drives busy is:
> +	 *
> +	 * drives |   I/O requests
> +	 * -------+----------------
> +	 *		1 |   1
> +	 *		2 |   2/1 + 2/2 = 3
> +	 *		3 |   3/1 + 3/2 + 3/3 = 5 1/2
> +	 *		4 |   4/1 + 4/2 + 4/3 + 4/4 = 8 1/3
> +	 *		n |   n * H(n)

I know you just moved this code. But: I don't buy this formula. Like at
all. Doesn't queuing and reordering entirely invalidate the logic here?

Perhaps more relevantly: Imo nodeBitmapHeapscan.c is the wrong place for
this. bufmgr.c maybe?

You also didn't touch
/*
 * How many buffers PrefetchBuffer callers should try to stay ahead of their
 * ReadBuffer calls by.  This is maintained by the assign hook for
 * effective_io_concurrency.  Zero means "never prefetch".
 */
int			target_prefetch_pages = 0;
which surely doesn't make sense anymore after these changes.

But do we even need that variable now?

> diff --git a/src/include/utils/guc.h b/src/include/utils/guc.h
> index dc167f9..57008fc 100644
> --- a/src/include/utils/guc.h
> +++ b/src/include/utils/guc.h
> @@ -26,6 +26,9 @@
>  #define MAX_KILOBYTES	(INT_MAX / 1024)
>  #endif
>  
> +/* upper limit for effective_io_concurrency */
> +#define MAX_IO_CONCURRENCY 1000
> +
>  /*
>   * Automatic configuration file name for ALTER SYSTEM.
>   * This file will be used to store values of configuration parameters
> @@ -256,6 +259,8 @@ extern int	temp_file_limit;
>  
>  extern int	num_temp_buffers;
>  
> +extern int	effective_io_concurrency;
> +

target_prefetch_pages is declared in bufmgr.h - that seems like a better
place for these.

Greetings,

Andres Freund


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