Re: [HACKERS] Logical tape pause/resume

On 12/21/2016 03:22 PM, Robert Haas wrote:
> On Wed, Dec 21, 2016 at 7:25 AM, Heikki Linnakangas <hlinnaka@iki.fi> wrote:
>> For now, barring objections, I'm going to commit the first patch. It seems
>> like a worthwhile simplification in any case, especially for Peter's
>> Parallel tuplesort patch set.
>
> Well, it removes more code than it adds.  That's definitely something.
> And saving memory per-empty-tape is good, too.  A few random comments:
>
> "seeked" is kind of a lame variable name.  How about "seekpos" or
> "newpos" or something like that?

Ok.

>      /*
>       * Even in minimum memory, use at least a MINORDER merge.  On the other
>       * hand, even when we have lots of memory, do not use more than a MAXORDER
> -     * merge.  Tapes are pretty cheap, but they're not entirely free.  Each
> -     * additional tape reduces the amount of memory available to build runs,
> -     * which in turn can cause the same sort to need more runs, which makes
> -     * merging slower even if it can still be done in a single pass.  Also,
> -     * high order merges are quite slow due to CPU cache effects; it can be
> -     * faster to pay the I/O cost of a polyphase merge than to perform a single
> -     * merge pass across many hundreds of tapes.
> +     * merge.  Tapes are pretty cheap, but they're not entirely free.
> High order
> +     * merges are quite slow due to CPU cache effects; it can be faster to pay
> +     * the I/O cost of a polyphase merge than to perform a single merge pass
> +     * across many hundreds of tapes.
>       */
>
> I think you could leave this comment as-is.  You haven't zeroed out
> the overhead of a tape, and I like those additional bits I crammed in
> there.  :-)

Yes, quite right. That was a mishap in rebasing, that change to remove 
the comment really belongs to the pause/resume patch rather than the 1st 
one.

Thanks for the review!

- Heikki