On Wed 06-12-17 06:15:35, Matthew Wilcox wrote: > On Wed, Dec 06, 2017 at 01:38:42PM +0100, Michal Hocko wrote: > > On Tue 05-12-17 18:25:21, Matthew Wilcox wrote: > > > -/* 14 pointers + two long's align the pagevec structure to a power of two */ > > > -#define PAGEVEC_SIZE 14 > > > +/* 15 pointers + header align the pagevec structure to a power of two */ > > > +#define PAGEVEC_SIZE 15 > > > > And now you have ruined the ultimate constant of the whole MM :p > > But seriously, I have completely missed that pagevec has such a bad > > layout. > > It's fun to go back into the historical tree and see why. > > First it was two 'int's and an array of 16 pointers. Marcelo noticed that > was three cachelines instead of two, so he shrank it to two shorts and > an array of 15 pointers: > > https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/include/linux/pagevec.h?id=afead7df5a05118052a238c54285e7119da65831 > > But then he found out that Pentium 2 and Pentium Pro sucked at 16-bit loads, > so he changed it to two longs and an array of 14 pointers: > > https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/include/linux/pagevec.h?id=6140f8a54db42320b1d05ce2680b5619210b88ad Yeah, i've done that exercise several times because 14 is just _strange_. I always had that feeling that we were trying to be too clever for minor things while larger ones just got unnotices... > I wonder what would have happened if he had benchmarked it with 'char' > instead of 'short'. I think I have a Pentium 2 in the basement somewhere; > perhaps I'll drag it out and fire it up. > > > Acked-by: Michal Hocko <mhocko@xxxxxxxx> > > Thanks! -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
