page fault scalability patch V16 [0/4]: redesign overview

Christoph Lameter
2005-01-28T20:35:11Z

Changes from V15->V16 of this patch: Complete Redesign.

An introduction to what this patch does and a patch archive can be found on
http://oss.sgi.com/projects/page_fault_performance. The archive also has a
combined patch.

The basic approach in this patchset is the same as used in SGI's 2.4.X
based kernels which have been in production use in ProPack 3 for a long time.

The patchset is composed of 4 patches (and was tested against 2.6.11-rc2-bk6
on ia64, i386 and x86_64):

1/4: ptep_cmpxchg and ptep_xchg to avoid intermittent zeroing of ptes

The current way of synchronizing with the CPU or arch specific
interrupts updating page table entries is to first set a pte
to zero before writing a new value. This patch uses ptep_xchg
and ptep_cmpxchg to avoid writing the zero for certain
configurations.

The patch introduces CONFIG_ATOMIC_TABLE_OPS that may be
enabled as a experimental feature during kernel configuration
if the hardware is able to support atomic operations and if
an SMP kernel is being configured. A Kconfig update for i386,
x86_64 and ia64 has been provided. On i386 this options is
restricted to CPUs better than a 486 and non PAE mode (that
way all the cmpxchg issues on old i386 CPUS and the problems
with 64bit atomic operations on recent i386 CPUS are avoided).

If CONFIG_ATOMIC_TABLE_OPS is not set then ptep_xchg and
ptep_xcmpxchg are realized by falling back to clearing a pte
before updating it.

The patch does not change the use of mm->page_table_lock and
the only performance improvement is the replacement of
xchg-with-zero-and-then-write-new-pte-value with an xchg with
the new value for SMP on some architectures if
CONFIG_ATOMIC_TABLE_OPS is configured. It should not do anything
major to VM operations.

2/4: Macros for mm counter manipulation

There are various approaches to handling mm counters if the
page_table_lock is no longer acquired. This patch defines
macros in include/linux/sched.h to handle these counters and
makes sure that these macros are used throughout the kernel
to access and manipulate rss and anon_rss. There should be
no change to the generated code as a result of this patch.

3/4: Drop the first use of the page_table_lock in handle_mm_fault

The patch introduces two new functions:

page_table_atomic_start(mm), page_table_atomic_stop(mm)

that fall back to the use of the page_table_lock if
CONFIG_ATOMIC_TABLE_OPS is not defined.

If CONFIG_ATOMIC_TABLE_OPS is defined those functions may
be used to prep the CPU for atomic table ops (i386 in PAE mode
may f.e. get the MMX register ready for 64bit atomic ops) but
are simply empty by default.

Two operations may then be performed on the page table without
acquiring the page table lock:

a) updating access bits in pte
b) anonymous read faults installed a mapping to the zero page.

All counters are still protected with the page_table_lock thus
avoiding any issues there.

Some additional statistics are added to /proc/meminfo to
give some statistics. Also counts spurious faults with no
effect. There is a surprisingly high number of those on ia64
(used to populate the cpu caches with the pte??)

4/4: Drop the use of the page_table_lock in do_anonymous_page

The second acquisition of the page_table_lock is removed
from do_anonymous_page and allows the anonymous
write fault to be possible without the page_table_lock.

The macros for manipulating rss and anon_rss in include/linux/sched.h
are changed if CONFIG_ATOMIC_TABLE_OPS is set to use atomic
operations for rss and anon_rss (safest solution for now, other
solutions may easily be implemented by changing those macros).

This patch typically yield significant increases in page fault
performance for threaded applications on SMP systems.

I have an additional patch that drops the page_table_lock for COW but that
raises a lot of other issues. I will post that patch separately and only
to linux-mm.