[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 <dimitry_at_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.
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