Re: Allow a per-tablespace effective_io_concurrency setting

Hi,

Please find attached a v2 of the patch. See below for changes.

On 02/09/2015 15:53, Andres Freund wrote:
>
> 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.
>

I removed these ifdefs, and the more problematic one in the struct.

>> +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?
>

Changing the formula, or changing the GUC to a number of pages to
prefetch is still discussed, so no change here.

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

Moved to bufmgr.c

> 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?

I slighty updated the comment. If the table doesn't belong to a
tablespace with an overloaded effective_io_concurrency, keeping this
pre-computed target_prefetch_pages can save a few cycles on each
execution, so I think it's better to keep it.

>
>> 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.
>

Moved to bufmgr.h

As said in a previous mail, I also fixed a problem when having settings
other than effective_io_concurrency for a tablespace lead to ignore the
regular effective_io_concurrency.

I also added the forgotten lock level (AccessExclusiveLock) for this
tablespace setting, which was leading to a failed assert during initdb.

Regards.

--
Julien Rouhaud
http://dalibo.com - http://dalibo.org