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Postings from November 2010
It looks like the perlio layer flushes to the unixio layer whenever
the perlio buffer fills up (assuming default layers with nothing
special pushed in between). That's for writes; for reads, replace
"flushes to" with "fills from." But the perlio buffer is hard-wired
to 4096 bytes, which seems odd both because it's small and because
it's a naked number with no documentation or justification for the
chosen size that I could find. As far as I can tell this specific
buffer size came in here:
http://perl5.git.perl.org/perl.git/commitdiff/bb9950b796df42e2f824a072ae878c87e977be20
and was never changed. That was 10 years ago, at a rather early stage
of PerlIO development (I *think* before layers as such even existed).
Apparently I am not the only one who has wondered about the small,
hard-coded size:
http://www.perlmonks.org/?node_id=692386
A larger buffer would result in less frequent flushes or fills and
presumably save some layer shuffling. This needs testing, which I
haven't gotten to yet. I have seen extra lines show up on a line-
buffered filehandle where the lines are longer than 4096 bytes, and a
larger buffer clearly allows longer lines in that situation. That I
have tested.
So what is a good size? I think a pretty safe option would be to make
it the larger of 4096 and BUFSIZ as below. That would leave it as is
where BUFSIZ is very small, but allows a better guess as to what's
appropriate for the local system if the maintainers of the relevant C
run-time have increased BUFSIZ to something larger. Another option
would be to use st.st_blksize.
I will try to do some proper benchmarking, but does anyone see offhand
any reason that the following wouldn't be an improvement?
--- perlio.c;-1 2010-11-04 19:23:21 -0500
+++ perlio.c 2010-11-04 21:35:30 -0500
@@ -4133,7 +4133,7 @@ PerlIOBuf_get_base(pTHX_ PerlIO *f)
if (!b->buf) {
if (!b->bufsiz)
- b->bufsiz = 4096;
+ b->bufsiz = BUFSIZ > 4096 ? BUFSIZ : 4096;
Newxz(b->buf,b->bufsiz, STDCHAR);
if (!b->buf) {
b->buf = (STDCHAR *) & b->oneword;
[end]
________________________________________
Craig A. Berry
mailto:craigberry@mac.com
"... getting out of a sonnet is much more
difficult than getting in."
Brad Leithauser
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