Re: Allow a per-tablespace effective_io_concurrency setting

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