Re: [HACKERS] Logical tape pause/resume

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?
    /*     * 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.  :-)

-- 
Robert Haas
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