When checking a performance change for will-it-scale scalability
mmap test [1], we found very high lock contention for spinlock of
percpu counter 'vm_committed_as':
94.14% 0.35% [kernel.kallsyms] [k] _raw_spin_lock_irqsave
48.21% _raw_spin_lock_irqsave;percpu_counter_add_batch;__vm_enough_memory;mmap_region;do_mmap;
45.91% _raw_spin_lock_irqsave;percpu_counter_add_batch;__do_munmap;
Actually this heavy lock contention is not always necessary. The
'vm_committed_as' needs to be very precise when the strict
OVERCOMMIT_NEVER policy is set, which requires a rather small batch
number for the percpu counter.
So keep 'batch' number unchanged for strict OVERCOMMIT_NEVER policy,
and enlarge it for not-so-strict OVERCOMMIT_ALWAYS and OVERCOMMIT_GUESS
policies.
Benchmark with the same testcase in [1] shows 53% improvement on a
8C/16T desktop, and 2097%(20X) on a 4S/72C/144T server. And for that
case, whether it shows improvements depends on if the test mmap size
is bigger than the batch number computed.
We tested 10+ platforms in 0day (server, desktop and laptop). If we
lift it to 64X, 80%+ platforms show improvements, and for 16X lift,
1/3 of the platforms will show improvements.
And generally it should help the mmap/unmap usage,as Michal Hocko
mentioned:
"
I believe that there are non-synthetic worklaods which would benefit
from a larger batch. E.g. large in memory databases which do large
mmaps during startups from multiple threads.
"
Note: There are some style complain from checkpatch for patch 3,
as sysctl handler declaration follows the similar format of sibling
functions
patch1: a cleanup for /proc/meminfo
patch2: a preparation patch which also improve the accuracy of
vm_memory_committed
patch3: remove the VM_WARN_ONCE for vm_committed_as underflow check
patch4: main change
This is against today's linux-mm git tree on github.
Please help to review, thanks!
- Feng
----------------------------------------------------------------
Changelog:
v4:
* Remove the VM_WARN_ONCE check for vm_committed_as underflow,
thanks to Qian Cai for finding and testing the warning
v3:
* refine commit log and cleanup code, according to comments
from Michal Hocko and Matthew Wilcox
* change the lift from 16X and 64X after test
v2:
* add the sysctl handler to cover runtime overcommit policy
change, as suggested by Andres Morton
* address the accuracy concern of vm_memory_committed()
from Andi Kleen
Feng Tang (4):
proc/meminfo: avoid open coded reading of vm_committed_as
mm/util.c: make vm_memory_committed() more accurate
mm/util.c: remove the VM_WARN_ONCE for vm_committed_as underflow check
mm: adjust vm_committed_as_batch according to vm overcommit policy
fs/proc/meminfo.c | 2 +-
include/linux/mm.h | 2 ++
include/linux/mman.h | 4 ++++
kernel/sysctl.c | 2 +-
mm/mm_init.c | 18 ++++++++++++++----
mm/util.c | 22 +++++++++++++---------
6 files changed, 35 insertions(+), 15 deletions(-)
--
2.7.4
