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library: dynamic buffer management even more efficient

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One recent patch to dynamic buffer management involved
over-allocating the buffer increase to lessen calls to
xrealloc. That was successful, but the actual increase
amount did not attempt to optimize size or alignments.

With this commit, we'll copy an approach recently used
by the top program and round up buffer sizes to 1 KiB.
More importantly, while buffers are quickly reaching a
KiB optimum multiple, no memcpy will ever be employed!

To illustrate just how effective top's algorithm would
be, just change the initial and subsequent allocations
from the current 1024 bytes to just a single byte then
add an fprintf.  Those one byte reallocations while on
the way to optimum buffer size will be a one-time cost
and won't represent any recurring performance penalty.

( gosh, that top program *must be* one fart smeller, )
( or was that a smart feller, i can't remember which )

Reference)s):
commit 6d605f521c
commit a45dace4b8

Signed-off-by: Jim Warner <james.warner@comcast.net>
This commit is contained in:
Jim Warner 2013-04-14 00:00:00 -05:00 committed by Craig Small
parent d16fd8e462
commit 95d0136281
1 changed files with 9 additions and 14 deletions
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23
proc/readproc.c
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@ -533,32 +533,27 @@ static void statm2proc(const char* s, proc_t *restrict P) {
}
static int file2str(const char *directory, const char *what, struct utlbuf_s *ub) {
#define readMAX 4096
#define buffMIN (tot_read + num + 1) // +1 for the '\0' delimiter
#define buffGRW (30 + (buffMIN * 5) / 4) // grow by more than 25%
char path[PROCPATHLEN], chunk[readMAX];
int fd, num, eof = 0, tot_read = 0;
#define buffGRW 1024
char path[PROCPATHLEN];
int fd, num, tot_read = 0;
/* on first use we preallocate a buffer of minimum size to emulate
former 'local static' behavior -- even if this read fails, that
buffer will likely soon be used for another subdirectory anyway */
buffer will likely soon be used for another subdirectory anyway
( besides, with this xcalloc we will never need to use memcpy ) */
if (ub->buf) ub->buf[0] = '\0';
else ub->buf = xcalloc((ub->siz = readMAX));
else ub->buf = xcalloc((ub->siz = buffGRW));
sprintf(path, "%s/%s", directory, what);
if (-1 == (fd = open(path, O_RDONLY, 0))) return -1;
while (!eof && 0 < (num = read(fd, chunk, readMAX))) {
if (ub->siz < buffMIN)
ub->buf = xrealloc(ub->buf, (ub->siz = buffGRW));
memcpy(ub->buf + tot_read, chunk, num);
while (0 < (num = read(fd, ub->buf + tot_read, ub->siz - tot_read))) {
tot_read += num;
eof = (num < readMAX);
if (tot_read < ub->siz) break;
ub->buf = xrealloc(ub->buf, (ub->siz += buffGRW));
};
ub->buf[tot_read] = '\0';
close(fd);
if (unlikely(tot_read < 1)) return -1;
return tot_read;
#undef readMAX
#undef buffMIN
#undef buffGRW
}