vfs cache: describe various optimization ideasHEADmain

While here report a sample result from running on Sapphire Rapids:

An access(2) loop slapped into will-it-scale, like so:
        while (1) {
                int error = access(tmpfile, R_OK);
                assert(error == 0);

                (*iterations)++;
        }

.. operating on /usr/obj/usr/src/amd64.amd64/sys/GENERIC/vnode_if.c

In operations per second:
lockless:	3462164
locked:		1362376

While the over 3.4 mln may seem like a big number, a critical look shows
it should be significantly higher.

A poor man's profiler, counting how many times given routine was sampled:
dtrace -w -n 'profile:::profile-4999 /execname == "a.out"/ {
@[sym(arg0)] = count(); } tick-5s { system("clear"); trunc(@, 40);
printa("%40a %@16d\n", @); clear(@); }'

[snip]
                    kernel`kern_accessat              231
           kernel`cpu_fetch_syscall_args              324
       kernel`cache_fplookup_cross_mount              340
                            kernel`namei              346
                    kernel`amd64_syscall              352
             kernel`tmpfs_fplookup_vexec              388
                             kernel`vput              467
                      kernel`vget_finish              499
                   kernel`lockmgr_unlock              529
                    kernel`lockmgr_slock              558
                    kernel`vget_prep_smr              571
                       kernel`vput_final              578
                           kernel`vdropl             1070
                           kernel`memcmp             1174
                     kernel`0xffffffff80             2080
                                     0x0             2231
                   kernel`copyinstr_smap             2492
                   kernel`cache_fplookup             9246

0 files changed, 0 insertions, 0 deletions