Re: [HACKERS] Parallel tuplesort (for parallel B-Tree index creation)

On 02/06/2018 10:14 PM, Peter Geoghegan wrote:
> On Tue, Feb 6, 2018 at 1:04 PM, Tomas Vondra
> <tomas.vondra@2ndquadrant.com> wrote:
>> Did you do a test with "-O0"? In my experience that makes valgrind tests
>> much more reliable and repeatable. Some time ago we've seen cases that
>> were failing for me but not for others, and I suspect it was due to me
>> using "-O0".
> 
> FWIW, I use -O1 when configure is run for Valgrind. I also turn off
> assertions (this is all scripted). According to the Valgrind manual:
> 
> "With -O1 line numbers in error messages can be inaccurate, although
> generally speaking running Memcheck on code compiled at -O1 works
> fairly well, and the speed improvement compared to running -O0 is
> quite significant. Use of -O2 and above is not recommended as Memcheck
> occasionally reports uninitialised-value errors which don’t really
> exist."
> 

OK, although I was suggesting the optimizations may actually have the
opposite effect - valgrind missing some of the invalid memory accesses
(until the compiler decides not use them for some reason, causing sudden
valgrind failures).

> The manual does also say that there might even be some problems with
> -O1 at a later point, but it sounds like it's probably worth it to me.
> Skink uses -Og, FWIW.
> 

I have little idea what -Og exactly means. It seems to be focused on
debugging experience, and so still does some of the optimizations. Which
I think would explain why skink was not detecting some of the failures
for a long time.

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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