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SubjectRe: Upcoming: Notifications, FS notifications and fsinfo()
On Wed, Apr 1, 2020 at 3:58 PM David Howells <dhowells@redhat.com> wrote:
>
> David Howells <dhowells@redhat.com> wrote:
>
> > > Attached patch applies against readfile patch.
> >
> > But doesn't actually do what Karel asked for. show_mountinfo() itself does
> > not give you what Karel asked for.

Not sure what you mean. I think it shows precisely the information
Karel asked for.

> Plus there's more information you need to
> > add to it.

The mountinfo format is extensible (see
Documentation/filesystems/proc.txt) so for example adding the change
counters would be simple.

> And arguably, it's worse than just reading /proc/mounts. If you get a
> notification that something changed (ie. you poll /proc/mounts or mount
> notifications gives you an overrun) you now have to read *every*
> /mountfs/*/info file. That is way more expensive.

fsinfo(2) will never be substantially cheaper than reading and parsing
/mnt/MNT_ID/info. In fact reading a large part of the mount table
using fsinfo(2) will be substantially slower than parsing
/proc/self/mountinfo (this doesn't actually do the parsing but that
would add a very small amount of overhead):

root@kvm:~# ./test-fsinfo-perf /tmp/a 30000
--- make mounts ---
--- test fsinfo by path ---
sum(mnt_id) = 960000
--- test fsinfo by mnt_id ---
sum(mnt_id) = 960000
--- test /proc/fdinfo ---
sum(mnt_id) = 960000
--- test mountfs ---
sum(mnt_id) = 960000
--- test mountinfo ---
sum(mnt_id) = 960000
For 30000 mounts, f= 154963us f2= 148337us p= 1803699us p2=
257019us; m= 53996us; p=11.6*f p=12.2*f2 p=7.0*p2 p=33.4*m
--- umount ---

Yes, that's 33 times faster!

Thanks,
Miklos
---
samples/vfs/test-fsinfo-perf.c | 91 +++++++++++++++++++++++++++++++++++++++--
1 file changed, 87 insertions(+), 4 deletions(-)

--- a/samples/vfs/test-fsinfo-perf.c
+++ b/samples/vfs/test-fsinfo-perf.c
@@ -339,6 +339,79 @@ static void get_id_by_mountfs(void)
} while (p = comma, *comma);
}

+static void get_id_by_mountinfo(void)
+{
+ unsigned int base_mnt_id, mnt_id, x;
+ ssize_t len;
+ char procfile[100], buffer[4096], *p, *nl;
+ int fd, fd2, mntfd;
+
+ /* Start off by reading the mount ID from the base path */
+ fd = open(base_path, O_PATH);
+ ERR(fd, "open/path");
+ sprintf(procfile, "/proc/self/fdinfo/%u", fd);
+ fd2 = open(procfile, O_RDONLY);
+ ERR(fd2, "open/proc");
+ len = read(fd2, buffer, sizeof(buffer) - 1);
+ ERR(len, "read");
+ buffer[len] = 0;
+ close(fd2);
+ close(fd);
+
+ p = buffer;
+ do {
+ nl = strchr(p, '\n');
+ if (nl)
+ *nl++ = '\0';
+ else
+ nl = NULL;
+
+ if (strncmp(p, "mnt_id:", 7) != 0)
+ continue;
+ p += 7;
+ while (isblank(*p))
+ p++;
+ /* Have to allow for extra numbers being added to the line */
+ if (sscanf(p, "%u", &base_mnt_id) != 1) {
+ fprintf(stderr, "Bad format %s\n", procfile);
+ exit(3);
+ }
+ break;
+
+ } while ((p = nl));
+
+ if (!p) {
+ fprintf(stderr, "Missing field %s\n", procfile);
+ exit(3);
+ }
+
+ if (0) printf("[B] %u\n", base_mnt_id);
+
+ mntfd = open("/proc/self/mountinfo", O_RDONLY);
+ ERR(mntfd, "/proc/self/mountinfo");
+
+ while ((len = read(mntfd, buffer, sizeof(buffer)))) {
+ ERR(len, "read/mountinfo");
+
+ for (p = buffer; p < buffer + len; p = nl + 1) {
+ nl = strchr(p, '\n');
+ if (!nl) {
+ fprintf(stderr, "error parsing mountinfo\n");
+ exit(3);
+ }
+ *nl = '\0';
+ if (sscanf(p, "%i %i", &mnt_id, &x) != 2) {
+ fprintf(stderr, "error parsing mountinfo\n");
+ exit(3);
+ }
+ if (x == base_mnt_id)
+ sum_check += x;
+ }
+ }
+
+ close(mntfd);
+}
+
static unsigned long duration(struct timeval *before, struct timeval *after)
{
unsigned long a, b;
@@ -354,8 +427,9 @@ int main(int argc, char **argv)
struct timeval f2_before, f2_after;
struct timeval p_before, p_after;
struct timeval p2_before, p2_after;
+ struct timeval m_before, m_after;
const char *path;
- unsigned long f_dur, f2_dur, p_dur, p2_dur;
+ unsigned long f_dur, f2_dur, p_dur, p2_dur, m_dur;

if (argc < 2) {
fprintf(stderr, "Format: %s <path> [nr_mounts]\n", argv[0]);
@@ -402,17 +476,26 @@ int main(int argc, char **argv)
ERR(gettimeofday(&p2_after, NULL), "gettimeofday");
printf("sum(mnt_id) = %lu\n", sum_check);

+ printf("--- test mountinfo ---\n");
+ sum_check = 0;
+ ERR(gettimeofday(&m_before, NULL), "gettimeofday");
+ get_id_by_mountinfo();
+ ERR(gettimeofday(&m_after, NULL), "gettimeofday");
+ printf("sum(mnt_id) = %lu\n", sum_check);
+
f_dur = duration(&f_before, &f_after);
f2_dur = duration(&f2_before, &f2_after);
p_dur = duration(&p_before, &p_after);
p2_dur = duration(&p2_before, &p2_after);
+ m_dur = duration(&m_before, &m_after);
//printf("fsinfo duration %10luus for %d mounts\n", f_dur, nr_mounts);
//printf("procfd duration %10luus for %d mounts\n", p_dur, nr_mounts);

- printf("For %7d mounts, f=%10luus f2=%10luus p=%10luus p2=%10luus; p=%.1f*f p=%.1f*f2 p=%.1f*p2\n",
- nr_mounts, f_dur, f2_dur, p_dur, p2_dur,
+ printf("For %7d mounts, f=%10luus f2=%10luus p=%10luus p2=%10luus; m=%10luus; p=%.1f*f p=%.1f*f2 p=%.1f*p2 p=%.1f*m\n",
+ nr_mounts, f_dur, f2_dur, p_dur, p2_dur, m_dur,
(double)p_dur / (double)f_dur,
(double)p_dur / (double)f2_dur,
- (double)p_dur / (double)p2_dur);
+ (double)p_dur / (double)p2_dur,
+ (double)p_dur / (double)m_dur);
return 0;
}
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