Re: cloning the kernel - why long time in "Resolving 313037 deltas"

From: Theodore Tso <tytso@mit.edu>
Date: Dec 19, 2006, 05:11 UTC
Message-ID: <20061219051108.GA29405@thunk.org>
In-Reply-To: <Pine.LNX.4.64.0612181906450.18171@xanadu.home>

On Mon, Dec 18, 2006 at 07:13:40PM -0500, Nicolas Pitre wrote:

Show 7 quoted lines

> Maybe.  However the mmap() may occur on section of the pack file which 
> has just been written to in order to write even more, always to the same 
> file.  On Linux this is fast because the mmap'd data is likely to still 
> be in the cache.
> 
> I guess this could be turned into a malloc()/read()/free() with no 
> trouble.

Actually, depending on the size of the chunk, even on Linux malloc/read/free can be faster than the mmap/munmap, because mmap/munmap calls involve page table manipulations, and even on Linux that is often slower or dead even with the memory copy involved with using malloc/read. Even when reading huge chunks of Canon Raw File data at a time, I found (experimentally) that it was no faster to use mmap() compared to read(). And for small chunks of data, malloc/read will definitely win out over mmap(), since the page table operations and resulting page faults completely trump the cost of copying the bytes from the page cache to the read() buffer.