Re: Increase BUFSIZ to 8192

On Wed, 2015-05-13 at 10:35 +0200, Hans Petter Selasky wrote:
> On 05/13/15 10:27, David Chisnall wrote:
> > On 13 May 2015, at 09:03, John-Mark Gurney <jmg_at_funkthat.com> wrote:
> >>
> >> Poul-Henning Kamp wrote this message on Tue, May 12, 2015 at 06:31 +0000:
> >>> --------
> >>> In message <20150512032307.GP37063_at_funkthat.com>, John-Mark Gurney writes:
> >>>
> >>>> Also, you'd probably see even better performance by increasing the
> >>>> size to 64k, [...]
> >>>
> >>> easy:
> >>> 	8K on 32bit
> >>> 	64k on 64bit
> >>
> >> Sounds good to me...  Just for people who care... I did a quick set of
> >> benchmarks on sha256.. This is using my preliminary patch to use sse4
> >> optimized sha256...  But this should be the same for others...
> >>
> >> The numbers in ministat output are the time in seconds it takes my
> >> 3.4GHz AMD A10-5700 APU running HEAD to process a 512MB file, so lower
> >> numbers are better..  I've processed them into easier to read format:
> >> BUFSIZ:	145MB/sec
> >> 8k:	193MB/sec
> >> 16k:	198MB/sec
> >> 64k:	202MB/sec
> >> 128k:	202MB/sec
> >> -t:	211MB/sec
> >
> > It looks like most of the benefit is gained at 16KB.  Did you try running the benchmark with something else running at the same time to see if there is any advantage in trashing the caches a bit less (simple case, what happens if you run two instances of the same benchmark at once)?
> >
> > I suspect that you’re about right anyway - I recently did some tests while playing with JavaScript FFI generation with a multithreaded process JavaScript environment calling out to OpenSSL to do SHA calculations and having each of 8 threads reading in 128KB chunks gave the fastest performance (Core i7, 4 cores + hyperthreading), with only a negligible gain over 64KB.  In all cases, the JavaScript implementation was significantly faster than the openssl tool, which used 8KB buffers.
> >
> 
> Hi,
> 
> You should also try this using an USB disk. The performance numbers 
> heavily depends on the hardware's interrupt moderation values.


All this discussion should be happening in phabricator, not the email
that announces the review on phab.  But, since it's now happening here,
I guess I'll transplant my comments from there to here...

There are 2125 occurrances of BUFSIZ in the base code (probably 95% of
them inappropriately used to size a local temp buffer or string). Do you
really want to perturb that much working tested software because it
makes md5 faster? How many of those occurrances are stack-allocated
variables and is it wise to allocate 8k buffers on the stack for all of
them? How about existing programs (not necessarily in base) that open
many streams concurrently... what will be the impact of a sudden 8x
increase in memory usage for them?

It seems to me that if libmd needs bigger buffers to perform well, it
should use setvbuf().

-- Ian