VFS scalability (was: [rfc] lockless pagecache)

[Nick Piggin <nickpiggin@yahoo.com.au>]

Just an interesting aside, when first testing the patch I was
using read(2) instead of nopage faults. I ran into some surprising
results there which I don't have the time to follow up at the
moment - it might be worth investigating if someone has the time,
regardless the state of the lockless pagecache work.

For the parallel workload as described in the parent post (but
read instead of fault), the vanilla kernel profile looks like
this:

 74453 total                                      0.0121
 25839 update_atime                              44.8594
 19595 _read_unlock_irq                         306.1719
 13025 do_generic_mapping_read                    5.5758
  9374 rw_verify_area                            29.2937
  1739 ia64_pal_call_static                       9.0573
  1567 default_idle                               4.0807
  1114 __copy_user                                0.4704
   848 _spin_lock                                 8.8333
   786 ia64_spinlock_contention                   8.1875
   246 ia64_save_scratch_fpregs                   3.8438
   187 ia64_load_scratch_fpregs                   2.9219
    16 file_read_actor                            0.0263
    15 fsys_bubble_down                           0.0586
    12 vfs_read                                   0.0170

This is with the filesystem mounted as noatime, so I can't work
out why update_atime is so high on the list. I suspect maybe a
false sharing issue with some other fields.

--
SUSE Labs, Novell Inc.
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