Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

On Mon, Jul 06, 2020 at 08:07:46AM +0200, Jan Ziak wrote:

On Sun, Jul 5, 2020 at 1:58 PM Greg KH [off-list ref] wrote:

quoted

It also is a measurable increase over reading just a single file.
Here's my really really fast AMD system doing just one call to readfile
vs. one call sequence to open/read/close:

        $ ./readfile_speed -l 1
        Running readfile test on file /sys/devices/system/cpu/vulnerabilities/meltdown for 1 loops...
        Took 3410 ns
        Running open/read/close test on file /sys/devices/system/cpu/vulnerabilities/meltdown for 1 loops...
        Took 3780 ns

370ns isn't all that much, yes, but it is 370ns that could have been
used for something else :)

I am curious as to how you amortized or accounted for the fact that
readfile() first needs to open the dirfd and then close it later.

quoted

From performance viewpoint, only codes where readfile() is called

multiple times from within a loop make sense:

dirfd = open();
for(...) {
  readfile(dirfd, ...);
}
close(dirfd);

dirfd can be AT_FDCWD or if the path is absolute, dirfd will be ignored,
so one does not have to open anything.  It would be an optimisation
if one wanted to read several files relating to the same process:

char dir[50];
sprintf(dir, "/proc/%d", pid);
dirfd = open(dir);
readfile(dirfd, "maps", ...);
readfile(dirfd, "stack", ...);
readfile(dirfd, "comm", ...);
readfile(dirfd, "environ", ...);
close(dirfd);

but one would not have to do that.