shaba/systemd
1
0
Fork 0
mirror of https://github.com/systemd/systemd.git synced 2025-03-19 22:50:17 +03:00
Code

basic/fileio: optimize buffer sizes in read_full_virtual_file()

Browse Source 
We'd proceed rather inefficiently: the initial buffer size was LINE_MAX/2,
i.e. only 1k. We can read 4k at the same cost.

Also, we'd try to allocate 1025, 2049, 4097 bytes, i.e. always one higher than
the power-of-two size. Effectively the allocation would be bigger, and we'd
waste the additional space. So let's allocate aligned to the power-of-two size.
size=4095, 8191, 16383, so we allocate 4k, 8k, 16k.
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2021-03-25 12:10:32 +01:00
parent ca79564309
commit a9899ff358
1 changed files with 14 additions and 14 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

28
src/basic/fileio.c
View File

@ -27,7 +27,8 @@
#include "string-util.h"
#include "tmpfile-util.h"
#define READ_FULL_BYTES_MAX (4U*1024U*1024U)
/* The maximum size of the file we'll read in one go. */
#define READ_FULL_BYTES_MAX (4U*1024U*1024U - 1)
int fopen_unlocked(const char *path, const char *options, FILE **ret) {
assert(ret);
@ -386,8 +387,10 @@ int read_full_virtual_file(const char *filename, char **ret_contents, size_t *re
/* Start size for files in /proc/ which usually report a file size of 0. (Files in /sys/ report a
* file size of 4K, which is probably OK for sizing our initial buffer, and sysfs attributes can't be
* larger anyway.) */
size = LINE_MAX / 2;
* larger anyway.)
*
* It's one less than 4k, so that the malloc() below allocates exactly 4k. */
size = 4095;
/* Limit the number of attempts to read the number of bytes returned by fstat(). */
n_retries = 3;
@ -414,10 +417,10 @@ int read_full_virtual_file(const char *filename, char **ret_contents, size_t *re
/* Double the buffer size */
if (size >= READ_FULL_BYTES_MAX)
return -E2BIG;
if (size > READ_FULL_BYTES_MAX / 2)
if (size > READ_FULL_BYTES_MAX / 2 - 1)
size = READ_FULL_BYTES_MAX; /* clamp to max */
else
size *= 2;
size = size * 2 + 1; /* Stay always one less than page size, so we malloc evenly */
}
buf = malloc(size + 1);
@ -466,16 +469,13 @@ int read_full_virtual_file(const char *filename, char **ret_contents, size_t *re
buf = TAKE_PTR(p);
}
if (!ret_size) {
/* Safety check: if the caller doesn't want to know the size of what we
* just read it will rely on the trailing NUL byte. But if there's an
* embedded NUL byte, then we should refuse operation as otherwise
* there'd be ambiguity about what we just read. */
if (memchr(buf, 0, n))
return -EBADMSG;
} else
if (ret_size)
*ret_size = n;
else if (memchr(buf, 0, n))
/* Safety check: if the caller doesn't want to know the size of what we just read it will
* rely on the trailing NUL byte. But if there's an embedded NUL byte, then we should refuse
* operation as otherwise there'd be ambiguity about what we just read. */
return -EBADMSG;
buf[n] = 0;
*ret_contents = TAKE_PTR(buf);