On Mon, Sep 15, 2014 at 11:25 AM, Peter Geoghegan <[email protected]> wrote:
> OK, I'll draft a patch for that today, including similar alterations
> to varstr_cmp() for the benefit of Windows and so on.
I attach a much simpler patch, that only adds an opportunistic
"memcmp() == 0" before a possible strcoll(). Both
bttextfastcmp_locale() and varstr_cmp() have the optimization added,
since there is no point in leaving anyone out for this part.
When this is committed, and I hear back from you on the question of
what to do about having an independent sortsupport state for
abbreviated tie-breakers (and possibly other issues of concern), I'll
produce a rebased patch with a single commit.
--
Peter Geoghegan
diff --git a/src/backend/utils/adt/varlena.c b/src/backend/utils/adt/varlena.c
new file mode 100644
index 7549542..53be8ec
*** a/src/backend/utils/adt/varlena.c
--- b/src/backend/utils/adt/varlena.c
*************** varstr_cmp(char *arg1, int len1, char *a
*** 1405,1410 ****
--- 1405,1438 ----
#endif
}
+ /*
+ * Fast path: Attempt an entirely opportunistic "memcmp() == 0".
+ *
+ * In general there is no reason to be optimistic about being able to
+ * resolve the string comparison in this way. There are many usage
+ * patterns in which this fast path will never be taken, and some in
+ * which it will frequently be taken. When it is frequently taken,
+ * clearly the optimization is worthwhile, since strcoll() is known to
+ * be very expensive. When it is not taken, the overhead is completely
+ * negligible, and perhaps even immeasurably small, even in the worst
+ * case. We have nothing to lose and everything to gain.
+ *
+ * These counter-intuitive performance characteristics are likely
+ * explained by the dominant memory latency cost when performing a
+ * strcoll(); in practice, this may give the compiler and CPU
+ * sufficient leeway to order things in a way that hides the overhead
+ * of useless memcmp() calls. Sorting is very probably bottlenecked on
+ * memory bandwidth, and so it's worth it to use a few arithmetic
+ * instructions on the off chance that it results in a saving in memory
+ * bandwidth. In effect, this makes use of a capacity that would
+ * otherwise go unused. The same memory needed to be read into CPU
+ * cache at this juncture anyway. Modern CPUs can execute hundreds of
+ * instructions in the time taken to fetch a single cache line from
+ * main memory.
+ */
+ if (len1 == len2 && memcmp(arg1, arg2, len1) == 0)
+ return 0;
+
#ifdef WIN32
/* Win32 does not have UTF-8, so we need to map to UTF-16 */
if (GetDatabaseEncoding() == PG_UTF8)
*************** bttextfastcmp_locale(Datum x, Datum y, S
*** 1842,1847 ****
--- 1870,1885 ----
len1 = VARSIZE_ANY_EXHDR(arg1);
len2 = VARSIZE_ANY_EXHDR(arg2);
+ /*
+ * Fast path: Attempt an entirely opportunistic memcmp() == 0, as
+ * explained within varstr_cmp()
+ */
+ if (len1 == len2 && memcmp(a1p, a2p, len1) == 0)
+ {
+ result = 0;
+ goto done;
+ }
+
if (len1 >= tss->buflen1)
{
pfree(tss->buf1);
*************** bttextfastcmp_locale(Datum x, Datum y, S
*** 1875,1880 ****
--- 1913,1919 ----
if (result == 0)
result = strcmp(tss->buf1, tss->buf2);
+ done:
/* We can't afford to leak memory here. */
if (PointerGetDatum(arg1) != x)
pfree(arg1);
--
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