Re: [HACKERS] increasing the default WAL segment size

On 1/4/17 10:03 PM, David Rowley wrote:
>> I recall taht pow(x, 2) and x * x result usually in the same assembly
>> code, but pow() can never be more optimal than a simple
>> multiplication. So I'd think that it is wiser to avoid it in this code
>> path. Documentation is missing for the new replication command
>> SHOW_WAL_SEG. Actually, why not just having an equivalent of the SQL
>> command and be able to query parameter values?
> This would probably be nicer written using a bitwise trick to ensure
> that no lesser significant bits are set. If it's a power of 2, then
> subtracting 1 should have all the lesser significant bits as 1, so
> binary ANDing to that should be 0. i.e no common bits.
>
> Something like:
>
> /* ensure segment size is a power of 2 */
> if ((wal_segment_size & (wal_segment_size - 1)) != 0)
> {
>    fprintf(stderr, _("%s: WAL segment size must be in the power of
> 2\n"), progname);
>    exit(1);
> }
>
> There's a similar trick in bitmapset.c for RIGHTMOST_ONE, so looks
> like we already have assumptions about two's complement arithmetic

Well, now that there's 3 places that need to do almost the same thing, I 
think it'd be best to just centralize this somewhere. I realize that's 
not going to save any significant amount of code, but it would make it 
crystal clear what's going on (assuming the excellent comment above 
RIGHTMOST_ONE was kept).
-- 
Jim Nasby, Data Architect, Blue Treble Consulting, Austin TX
Experts in Analytics, Data Architecture and PostgreSQL
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