page fault scalability patch V12 [0/7]: Overview and performance

Christoph Lameter
2004-12-01T23:41:06Z

Changes from V11->V12 of this patch:
- dump sloppy_rss in favor of list_rss (Linus' proposal)
- keep up against current Linus tree (patch is based on 2.6.10-rc2-bk14)

This is a series of patches that increases the scalability of
the page fault handler for SMP. Here are some performance results
on a machine with 512 processors allocating 32 GB with an increasing
number of threads (that are assigned a processor each).

Without the patches:
Gb Rep Threads User System Wall flt/cpu/s fault/wsec
32 3 1 1.416s 138.165s 139.050s 45073.831 45097.498
32 3 2 1.397s 148.523s 78.044s 41965.149 80201.646
32 3 4 1.390s 152.618s 44.044s 40851.258 141545.239
32 3 8 1.500s 374.008s 53.001s 16754.519 118671.950
32 3 16 1.415s 1051.759s 73.094s 5973.803 85087.358
32 3 32 1.867s 3400.417s 117.003s 1849.186 53754.928
32 3 64 5.361s 11633.040s 197.034s 540.577 31881.112
32 3 128 23.387s 39386.390s 332.055s 159.642 18918.599
32 3 256 15.409s 20031.450s 168.095s 313.837 37237.918
32 3 512 18.720s 10338.511s 86.047s 607.446 72752.686

With the patches:
Gb Rep Threads User System Wall flt/cpu/s fault/wsec
32 3 1 1.451s 140.151s 141.060s 44430.367 44428.115
32 3 2 1.399s 136.349s 73.041s 45673.303 85699.793
32 3 4 1.321s 129.760s 39.027s 47996.303 160197.217
32 3 8 1.279s 100.648s 20.039s 61724.641 308454.557
32 3 16 1.414s 153.975s 15.090s 40488.236 395681.716
32 3 32 2.534s 337.021s 17.016s 18528.487 366445.400
32 3 64 4.271s 709.872s 18.057s 8809.787 338656.440
32 3 128 18.734s 1805.094s 21.084s 3449.586 288005.644
32 3 256 14.698s 963.787s 11.078s 6429.787 534077.540
32 3 512 15.299s 453.990s 5.098s 13406.321 1050416.414

For more than 8 cpus the page fault rate increases by orders
of magnitude. For more than 64 cpus the improvement in performace
is 10 times better.

The performance increase is accomplished by avoiding the use of the
page_table_lock spinlock (but not mm->mmap_sem!) through new atomic
operations on pte's (ptep_xchg, ptep_cmpxchg) and on pmd and pgd's
(pgd_test_and_populate, pmd_test_and_populate).

The page table lock can be avoided in the following situations:

1. An empty pte or pmd entry is populated

This is safe since the swapper may only depopulate them and the
swapper code has been changed to never set a pte to be empty until the
page has been evicted. The population of an empty pte is frequent
if a process touches newly allocated memory.

2. Modifications of flags in a pte entry (write/accessed).

These modifications are done by the CPU or by low level handlers
on various platforms also bypassing the page_table_lock. So this
seems to be safe too.

One essential change in the VM is the use of pte_cmpxchg (or its
generic emulation) on page table entries before doing an
update_mmu_change without holding the page table lock. However, we do
similar things now with other atomic pte operations such as
ptep_get_and_clear and ptep_test_and_clear_dirty. These operations
clear a pte *after* doing an operation on it. The ptep_cmpxchg as used
in this patch operates on an *cleared* pte and replaces it with a pte
pointing to valid memory. The effect of this change on various
architectures has to be thought through. Local definitions of
ptep_cmpxchg and ptep_xchg may be necessary.

For IA64 an icache coherency issue may arise that potentially requires
the flushing of the icache (as done via update_mmu_cache on IA64) prior
to the use of ptep_cmpxchg. Similar issues may arise on other platforms.

The patch introduces a split counter for rss handling to avoid atomic
operations and locks currently necessary for rss modifications. In
addition to mm->rss, tsk->rss is introduced. tsk->rss is defined to be
in the same cache line as tsk->mm (which is already used by the fault
handler) and thus tsk->rss can be incremented without locks
in a fast way. The cache line does not need to be shared between
processors in the page table handler.

A tasklist is generated for each mm (rcu based). Values in that list
are added up to calculate rss or anon_rss values.

The patchset is composed of 7 patches:

1/7: Avoid page_table_lock in handle_mm_fault

This patch defers the acquisition of the page_table_lock as much as
possible and uses atomic operations for allocating anonymous memory.
These atomic operations are simulated by acquiring the page_table_lock
for very small time frames if an architecture does not define
__HAVE_ARCH_ATOMIC_TABLE_OPS. It also changes the swapper so that a
pte will not be set to empty if a page is in transition to swap.

If only the first two patches are applied then the time that the
page_table_lock is held is simply reduced. The lock may then be
acquired multiple times during a page fault.

2/7: Atomic pte operations for ia64

3/7: Make cmpxchg generally available on i386

The atomic operations on the page table rely heavily on cmpxchg
instructions. This patch adds emulations for cmpxchg and cmpxchg8b
for old 80386 and 80486 cpus. The emulations are only included if a
kernel is build for these old cpus and are skipped for the real
cmpxchg instructions if the kernel that is build for 386 or 486 is
then run on a more recent cpu.

This patch may be used independently of the other patches.

4/7: Atomic pte operations for i386

A generally available cmpxchg (last patch) must be available for
this patch to preserve the ability to build kernels for 386 and 486.

5/7: Atomic pte operation for x86_64

6/7: Atomic pte operations for s390

7/7: Split counter implementation for rss
Add tsk->rss and tsk->anon_rss. Add tasklist. Add logic
to calculate rss from tasklist.

There are some additional outstanding performance enhancements that are
not available yet but which require this patch. Those modifications
push the maximum page fault rate from ~ 1 mio faults per second as
shown above to above 3 mio faults per second.

The last editions of the sloppy rss, atomic rss and deferred rss patch
will be posted to linux-ia64 for archival purpose.

Signed-off-by: Christoph Lameter <clameter@sgi.com>