>>>>> "Jan" == Jan Hudec <bulb@ucw.cz> writes: Jan> On Fri, Oct 18, 2002 at 04:26:44PM +0300, Momchil Velikov wrote: Jan> Yes, it should. But then message queue is most appropriate since it can Jan> also pass the actual data along. That's what a file descriptor with Jan> appropriately implemented poll is. >> >> >> >> >> >> It involves copying. Theoretically it is possible to have the read >> >> >> system call avoid copying for whole overwriten pages (by exchanging >> ^^^^^ >> >> >> >> page table entries (and flushing TLBs :-( )), but this may work well >> >> >> on some systems, work not so well on others and not work AT ALL when >> >> >> the source buffer is actually device memory. >> >> Jan> No, it won't work, because the read buffer would have to have same Jan> alignment as the data in cache. It is not a problem as there's an easy and portable way to obtain page-aligned user buffers. >> >> >> >> I do not understand this ? What do you mean ? What won't work ? >> Jan> You said, that read could possibly avoid copying by replacing PTEs. But Jan> it counldn't unless the data is appropriately aligned. That is if you Jan> read a whole page, starting at page-aligned offset and the buffer is Jan> page-aligned, then it will work, but in most other cases it won't. Jan> (Exact condition is: if buffer % PAGE_SIZE == offset % PAGE_SIZE, it Jan> could be done, otherwise not.) >> >> Every buffer larger than or equal to two page sizes contains at least >> one page aligned on page size boundary, i.e. at least one page frame. >> That page can be remapped. That's why I said "whole overwriten pages". Jan> No, it does not. Uh, oh, yes, a little brainfart from my side. Anyway, while this restriction is easy to be avoided, it is still another argument against read based implementation. QED. ~velco -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
