On Tue, Aug 17, 2010 at 10:45 AM, Kostik Belousov <kostik...@gmail.com>wrote:
> [Cc: list sanitized] > > On Tue, Aug 17, 2010 at 05:28:08PM +0200, Dimitry Andric wrote: > > On 2010-08-16 10:55, Dag-Erling Sm??rgrav wrote: > > > Dimitry Andric <dimi...@andric.com> writes: > > >> - Uses plain file descriptors instead of struct FILE, since the > > >> buffering is done manually anyway, and it makes it easier to support > > >> gzip and bzip2. > > > It might be worth a shot adding mmap(2) support as well, i.e. when > > > processing an uncompressed regular file, try to mmap(2) it first, and > if > > > that fails, fall back to the plain buffered read(2) method. > > > > I added a simple mmap to grep, and time-trialed it, but the mmap version > > was somewhat slower than the regular version. I understood from Kostik > > Belousov that readahead does not work properly with mmap, and it should > > not be used for "one-time" reads. > This is not exactly what I said. I argue that read-ahead implemented > by vm_faul() is much less efficient that buffer clustering. Also, > the cost of setting user mapping for the one time read is also non-trivial. > The conclusion is right, it is better to use read(2) for one-time read. > The mapping (and unmapping) costs should be relatively small if the contents of the file can be prefaulted using 2/4MB pages. In such cases, we still touch every struct vm_page in the 2/4MB region, but we only create and destroy one PTE and PV entry, and perform a single INVLPG. Alan _______________________________________________ freebsd-current@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-current To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
