On Mon, 4 Jun 2007 16:12:35 +0200 Ingo Molnar <[EMAIL PROTECTED]> wrote:
> > * Eric Dumazet <[EMAIL PROTECTED]> wrote: > > > O(1) lookup doesnt imply it needs to be super-fast. You make a > > confusion about this. > > Davide has written many good speedups for the kernel and he is one of > the best scalability experts the Linux kernel has today. You could learn > from Davide a thing or two, instead of lecturing him about O(1) ... > I know who is Davide, thank you :) > For example, the recent futex.c changes you did in commit 34f01cc1 are, > and unfortunately there's no better word i can find: plain disgusting. > You apparently have plopped the 'fshared' code into the existing logic > via conditionals and have blown up the complexity of the functions for > no good reason - instead of neatly separating them out. You have added > _33_ (thirty-three!) new 'if' branches to futex.c! The feature you > introduced is nice and useful, but for heaven's sake please work on > cleanliness of your code some more and undo that colossal damage ... > preferably before working on other areas of the kernel. > This code took the normal path for inclusion and discussion. If you find it so horrible, you should complained before. Fact is that you Acked it :) If you wanted to make a joke, I find it quite misplaced. > > O(128) is still O(1) for instance. Having to search a bit in a PAGE is > > a sensible compromise, if we dont add overhead on each fget() calls. > > hm, i'm not sure what you are talking about here. Look at Davide's stuff > - it's clearly not O(128)... I am talking about my suggestion to use a bitmap search limited to one page. So I named it O(128), this clearly should be labeled O(PAGE_SIZE/L1_CACHE_BYTES) > > > You add conditional branches on very hot spots. > > > > When you open/close a file, you need to access previous and next > > cells, so you need 3 cache lines, exactly like current *legacy* code. > > (one for file pointer, one on each bitmap flags(open/close_on_exec) ) > > the fd spaces will be separated _no matter what_, that is a physical > inevitability of the ABI in question. Whether you hide that into 'extra > complexity by trying to bend bitmaps in a way that the new users dont > need' or do it explicitly like Davide, i'll go for the explicit > separation. Davide's approach, besides being cleaner, simpler and faster > also has the advantage of enabling the possible demoting of the 'legacy' > fd space in the future. Or demoting the 'new' fd space in the future, if > the interface does not take off. > > Ingo > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
