Re: [PATCH] Teach "git add" and friends to be paranoid

From: Zygo Blaxell <zblaxell@gibbs.hungrycats.org>
Date: Feb 22, 2010, 17:31 UTC
Message-ID: <20100222173122.GG11733@gibbs.hungrycats.org>
In-Reply-To: <alpine.LFD.2.00.1002221033120.1946@xanadu.home>

On Mon, Feb 22, 2010 at 10:40:59AM -0500, Nicolas Pitre wrote:

Show 14 quoted lines

> On Sun, 21 Feb 2010, Junio C Hamano wrote:
> > Dmitry Potapov <dpotapov@gmail.com> writes:
> > > But overall the outcome is clear -- read() is always a winner.
> > 
> > "... a winner, below 128kB; above that the difference is within noise and
> > measurement error"?
> 
> read() is not always a winner.  A read() call will always have the data 
> duplicated in memory.  Especially with large files, it is more efficient 
> on the system as a whole to mmap() a 50 MB file rather than allocating 
> an extra 50 MB of anonymous memory that cannot be paged out (except to 
> the swap file which would be yet another data duplication).  With mmap() 
> when there is memory pressure the read-only mapped memory is simply 
> dropped with no extra IO.

That holds if you're comparing read() and mmap() of the entire file as a single chunk, instead of in fixed-size chunks at the sweet spot between syscall overhead and CPU cache size.

If you're read()ing a chunk at a time into a fixed size buffer, and doing sha1 and deflate in chunks, the data should be copied once into CPU cache, processed with both algorithms, and replaced with new data from the next chunk. The data will be copied from the page cache instead of directly mapped, which is a small overhead, but setting up the page map in mmap() also a small overhead, so you have to use benchmarks to know which of the overheads is smaller. It might be that there's no one answer that applies to all CPU configurations.

If you're doing mmap() and sha1 and deflate of a 50MB file in two separate passes that are the same size as the file, you load 50MB of data into CPU cache at least twice, you get two sets of associated things like TLB misses, and if the file is very large, you page it from disk twice. So it might make sense to process in chunks regardless of read() vs mmap() fetching the data.

If you're malloc()ing 50MB, you're wasting memory and CPU bandwidth making up pages full of zeros before you've even processed the first byte. I don't see how that could ever be faster for large file cases.