Re: SimpleLruTruncate() mutual exclusion

> On Sun, Nov 17, 2019 at 10:14:26PM -0800, Noah Misch wrote:
>
> Though I did reproduce this bug, I'm motivated by the abstract problem more
> than any particular way to reproduce it.  Commit 996d273 inspired me; by
> removing a GetCurrentTransactionId(), it allowed the global xmin to advance at
> times it previously could not.  That subtly changed the concurrency
> possibilities.  I think safe, parallel SimpleLruTruncate() is difficult to
> maintain and helps too rarely to justify such maintenance.  That's why I
> propose eliminating the concurrency.

Sure, I see the point and the possibility for the issue itself, but of
course it's easier to reason about an issue I can reproduce :)

> I wonder about performance in a database with millions of small relations,
> particularly considering my intent to back-patch this.  In such databases,
> vac_update_datfrozenxid() can be a major part of the VACUUM's cost.  Two
> things work in our favor.  First, vac_update_datfrozenxid() runs once per
> VACUUM command, not once per relation.  Second, Autovacuum has this logic:
>
> 	 * ... we skip
> 	 * this if (1) we found no work to do and (2) we skipped at least one
> 	 * table due to concurrent autovacuum activity.  In that case, the other
> 	 * worker has already done it, or will do so when it finishes.
> 	 */
> 	if (did_vacuum || !found_concurrent_worker)
> 		vac_update_datfrozenxid();
>
> That makes me relatively unworried.  I did consider some alternatives:

Btw, I've performed few experiments with parallel vacuuming of 10^4
small tables that are taking some small inserts, the results look like
this:

    # with patch
    # funclatency -u bin/postgres:vac_update_datfrozenxid

         usecs               : count     distribution
             0 -> 1          : 0        |                                        |
             2 -> 3          : 0        |                                        |
             4 -> 7          : 0        |                                        |
             8 -> 15         : 0        |                                        |
            16 -> 31         : 0        |                                        |
            32 -> 63         : 0        |                                        |
            64 -> 127        : 0        |                                        |
           128 -> 255        : 0        |                                        |
           256 -> 511        : 0        |                                        |
           512 -> 1023       : 3        |***                                     |
          1024 -> 2047       : 38       |****************************************|
          2048 -> 4095       : 15       |***************                         |
          4096 -> 8191       : 15       |***************                         |
          8192 -> 16383      : 2        |**                                      |

    # without patch
    # funclatency -u bin/postgres:vac_update_datfrozenxid

         usecs               : count     distribution
             0 -> 1          : 0        |                                        |
             2 -> 3          : 0        |                                        |
             4 -> 7          : 0        |                                        |
             8 -> 15         : 0        |                                        |
            16 -> 31         : 0        |                                        |
            32 -> 63         : 0        |                                        |
            64 -> 127        : 0        |                                        |
           128 -> 255        : 0        |                                        |
           256 -> 511        : 0        |                                        |
           512 -> 1023       : 5        |****                                    |
          1024 -> 2047       : 49       |****************************************|
          2048 -> 4095       : 11       |********                                |
          4096 -> 8191       : 5        |****                                    |
          8192 -> 16383      : 1        |                                        |

In general it seems that latency tends to be a bit higher, but I don't
think it's significant.