Re: Request for detailed documentation of git pack protocol

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: May 15, 2009, 20:29 UTC
Message-ID: <alpine.LFD.2.01.0905151324280.3343@localhost.localdomain>
In-Reply-To: <4A0C3282.4000101@op5.se>

On Thu, 14 May 2009, Andreas Ericsson wrote:

Show 12 quoted lines

> Shawn O. Pearce wrote:
> > 
> > In loopback mode for local file URIs, it may become an issue.  C Git
> > is just getting lucky by the pipe size I think.  Though I thought I
> > read somewhere yesterday pipe FIFOs in Linux were being allocated
> > at 512 bytes, not one system page.  Of course other systems could
> > allocate whatever size they want too, and may allocate something
> > below the 2952 minimum, and we'd most likely see a deadlock on them.
> > 
> 
> Linux allocates one page 4096 bytes for a FIFO. 512 is the maximum
> size guaranteed by POSIX to result in an atomic write.

Actually, modern Linux will allocate up to 16 pages (PIPE_BUFFERS), but they may not all be filled - we coalesce small writes only if the end result fits entirely into a page. So the maximum buffer is 16*PAGE_SIZE, and the minimum buffer space (assuming regular "write()" system calls) is something like 16*(PAGE_SIZE/2+1).

But yeah, POSIX allows for much smaller buffers.

			Linus