On Sat, 28 Apr 2007 11:21:17 +0100 Alan Cox <[EMAIL PROTECTED]> wrote:
> > > Also remember that even if you do larger pages by using virtual pairs or > > > quads of real pages because it helps on some systems you end up needing > > > the same sized sglist as before so you don't make anything worse for > > > half-assed controllers as you get the same I/O size providing they have > > > the minimal 2 or 4 sg list entries (and those that don't are genuinely > > > beyond saving and nowdays very rare) > > > > > > > Could you expand on that a bit please? I don't get it. > > Put a 16K "page" into the page cache physically and you need to allocate > 1 sg entry and you get a clear benefit, IFF you can allocate the pages. > > Put a 16K "page" into the page cache made up of 4 x real 4K pages which > are not physically contiguous and you need 4 sg list entries - which is > no worse than if you were using 4K pages > > 4 per 16K page cache "logcial page" -> 4 per 16K > > 1 per 4K physical page for 4K page cache -> 4 per 16K > > The only ugly case for the latter is if you are reading something like a > 16K page ext3fs from an old IA64 box onto a real computer and you have a > controller with insufficient sg list entries to read a 16K logical page. > At that point the block layer is going to have kittens. > OK. But all (both) the proposals we're (ahem) discussing do involve 4x physically contiguous pages going into those four contiguous pagecache slots. So we're improving things for the half-assed controllers, aren't we? - 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/
