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[PATCH] readhistfile: avoid thousands of lseek(2) syscalls via ftell()
[please cc me in replies, as I’m not subscribed to this list]
Before this change, zsh startup time was dominated by lseek(2) system calls on
the history file, as shown by strace -c:
time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
97,35 1,112890 1 697153 1 lseek
0,99 0,011314 2 5277 read
[…]
This change keeps track of read bytes and the position within the file, removing
all of these system calls.
I verified correctness of the change by comparing fpos with ftell(in) in every
loop iteration.
From 55ca0751f11432efab667451cc9b63ec0becc038 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <stapelberg@xxxxxxxxxx>
Date: Fri, 7 Feb 2020 08:41:26 +0100
Subject: [PATCH] readhistfile: avoid thousands of lseek(2) syscalls via
ftell()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Before this change, zsh startup time was dominated by lseek(2) system calls on
the history file, as shown by strace -c:
time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
97,35 1,112890 1 697153 1 lseek
0,99 0,011314 2 5277 read
[…]
This change keeps track of read bytes and the position within the file, removing
all of these system calls.
I verified correctness of the change by comparing fpos with ftell(in) in every
loop iteration.
---
Src/hist.c | 13 ++++++++-----
1 file changed, 8 insertions(+), 5 deletions(-)
diff --git a/Src/hist.c b/Src/hist.c
index 5281e8718..12eef5294 100644
--- a/Src/hist.c
+++ b/Src/hist.c
@@ -2575,10 +2575,11 @@ resizehistents(void)
}
static int
-readhistline(int start, char **bufp, int *bufsiz, FILE *in)
+readhistline(int start, char **bufp, int *bufsiz, FILE *in, int *readbytes)
{
char *buf = *bufp;
if (fgets(buf + start, *bufsiz - start, in)) {
+ *readbytes += strlen(buf + start);
int len = start + strlen(buf + start);
if (len == start)
return -1;
@@ -2588,7 +2589,7 @@ readhistline(int start, char **bufp, int *bufsiz, FILE *in)
return -1;
*bufp = zrealloc(buf, 2 * (*bufsiz));
*bufsiz = 2 * (*bufsiz);
- return readhistline(len, bufp, bufsiz, in);
+ return readhistline(len, bufp, bufsiz, in, readbytes);
}
}
else {
@@ -2596,7 +2597,7 @@ readhistline(int start, char **bufp, int *bufsiz, FILE *in)
if (len > 1 && buf[len - 2] == '\\') {
buf[--len - 1] = '\n';
if (!feof(in))
- return readhistline(len, bufp, bufsiz, in);
+ return readhistline(len, bufp, bufsiz, in, readbytes);
}
}
return len;
@@ -2616,7 +2617,7 @@ readhistfile(char *fn, int err, int readflags)
short *words;
struct stat sb;
int nwordpos, nwords, bufsiz;
- int searching, newflags, l, ret, uselex;
+ int searching, newflags, l, ret, uselex, readbytes;
if (!fn && !(fn = getsparam("HISTFILE")))
return;
@@ -2658,13 +2659,15 @@ readhistfile(char *fn, int err, int readflags)
} else
searching = 0;
+ fpos = ftell(in);
+ readbytes = 0;
newflags = HIST_OLD | HIST_READ;
if (readflags & HFILE_FAST)
newflags |= HIST_FOREIGN;
if (readflags & HFILE_SKIPOLD
|| (hist_ignore_all_dups && newflags & hist_skip_flags))
newflags |= HIST_MAKEUNIQUE;
- while (fpos = ftell(in), (l = readhistline(0, &buf, &bufsiz, in))) {
+ while (fpos += readbytes, readbytes = 0, (l = readhistline(0, &buf, &bufsiz, in, &readbytes))) {
char *pt;
int remeta = 0;
--
2.25.0
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