Performance of SheevaPlug on 8-stable

[Mark Tinguely]

On Sun, Feb 5, 2012 at 11:48 PM, Ian Lepore
<deletions>
> Okay, I've just confirmed that the problem as I described it above still
> exists in 9.0 ...
>
>        # uname -a
>        FreeBSD  9.0-RELEASE FreeBSD 9.0-RELEASE #7: Mon Feb  6 04:38:59
>        UTC 2012
>        root at revolution.hippie.lan:/usr/obj/arm.arm/usr/src/sys/TFLEX
>        arm
>
>        # /usr/tsc/bin/testsimple
>        Elapsed 0.782453
>        # /usr/tsc/bin/testsimple
>        Elapsed 0.782539
>
>        # cat /usr/tsc/bin/testsimple >/dev/null
>
>        # /usr/tsc/bin/testsimple
>        Elapsed 10.090336
>        # /usr/tsc/bin/testsimple
>        Elapsed 10.090611
>
> The testsimple program is just a little loop that repeatedly assigns a
> volatile variable (had to fool the optimizer into generating some code).
> I statically linked it to avoid any variability based on the race
> between paging and readahead when loading shared libs.  I'm just showing
> here that the base problem still exists:  when executable pages get into
> the vfs buffer cache they (semi-)permanently lose their i-cache bit.
>
> I also applied my patches from the start of this thread and re-tested
> and they appear to still be a usable workaround for the problem in 9.
> But they are just a workaround, we need to figure out a real fix for
> this.  (I had to enable the #if in ffs_read() as well as the one in
> ffs_write() for this test, since my quick 'cat' test is reading the file
> to get it into the cache.)
>
> -- Ian


I dug out my files from that era.

I was looking at the cases where the page is marked executable and writable:

 1) were they shared or single mappings?
 2) are all the shared mappings executable?
 3) was the page mapped several time in one process?
 4) was the all the writers a kernel process?
 5) was there many kernel mappings

I was think something like (pardon the bad formatting)

 if (exec && (exec == entries + kentries) &&
#ifndef test1
   (writable == 0 || (writable == kwritable)) &&
#else
   (writable == 0 || (writable == 1 && kwritable == 1)) &&
#endif
   (ncaches == 0)) {
      return;  /* bypass the cache fix routine */
}

 this test is just before the comment that says

                /*
                 * check if the user duplicate mapping has
                 * been removed.
                 */

I never felt brave enough with this test though.

--Mark.