On Fri, 2006-09-01 at 09:29 -0700, Dave Hansen wrote: > > 3) The page-has-a-writable-mapping (PG_writable) bit is set when the > > first writable pte for a page is established. The page needs to have a > > different state if a writable pte exists compared to a read-only page. > > The alternative without the page bit would be to do the state change > > every time a writable pte is established or to search all ptes of a > > given page. Both have performance implications. > > What are the performance implications? Do they completely erase any > performance gains that these patches might have given in the first > place? Has there been any evaluation of these other two alternatives? > As I understand it, carrying out this performance analysis would be very > difficult for most of the kernel community to perform. It seemed obvious to me that anything else than checking a bit is way to expensive. I never implemented nor measured any of the alternatives. The alternative to do the state change every time a writable pte is established can be implemented without too much trouble. Perhaps I will give it a try next week. > Keeping a nice count of the number of writable PTEs sounds like > something that might be generally useful. Could we split > page->_mapcount to keep track of r/o and r/w ptes separately? Or, > perhaps a single bit in it can be utilized to replace PG_writable, > instead. Yes, that would be really useful for the writable ptes. But I have the feeling that the actual implementation of it will be tricky. -- blue skies, Martin. Martin Schwidefsky Linux for zSeries Development & Services IBM Deutschland Entwicklung GmbH "Reality continues to ruin my life." - Calvin.
