On Wed, 27 Jun 2007, Davide Libenzi wrote: > On Wed, 27 Jun 2007, Hugh Dickins wrote: > > > In honesty, I should add that I dislike and distrust Davide's > > MAP_NOZERO very much indeed! Would much rather leave my cpus > > spending a little time in clear_page(). A uid in struct page > > (though I'm sure we could find somewhere to tuck it away) - > > the horror, the horror! But I've so far failed to find a killer > > argument against it, and am hoping for someone else to do so. > > Little time? Please, do not trust me. Start oprofile and run a kernel > build. Look, I'm not even talking about som micro benchmark explicitly > built to exploit the thing. A kernel build. > You will find clear_page to be the *1st* kernel entry after cc1 and as. > That is bad for two reasons. The time it spends in there, and the cache it > blows.
I don't doubt that it shows real benefits; but dangerously cutting corners usually shows benefits too. Relying on a uid at this level feels very wrong to me - but as I said, I've not found a killer argument against it. And we both know that clear_page features so high in part because it's bringing cachelines in from the cold, which are about to be accessed again by userspace; so it's often not so bad as it appears. Though I probably wouldn't be citing that argument if we were talking about offloading clear_page to another engine. Hugh - 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/
