Re: [PATCH 2/2] Make batch size for memory accounting configured according to size of memory

[Ric Mason <ric.masonn@gmail.com>]

Hi Tim,
On 04/30/2013 01:12 AM, Tim Chen wrote:
> Currently the per cpu counter's batch size for memory accounting is
> configured as twice the number of cpus in the system.  However,
> for system with very large memory, it is more appropriate to make it
> proportional to the memory size per cpu in the system.
>
> For example, for a x86_64 system with 64 cpus and 128 GB of memory,
> the batch size is only 2*64 pages (0.5 MB).  So any memory accounting
> changes of more than 0.5MB will overflow the per cpu counter into
> the global counter.  Instead, for the new scheme, the batch size
> is configured to be 0.4% of the memory/cpu = 8MB (128 GB/64 /256),

If large batch size will lead to global counter more inaccurate?

> which is more inline with the memory size.
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>   mm/mmap.c  | 13 ++++++++++++-
>   mm/nommu.c | 13 ++++++++++++-
>   2 files changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 0db0de1..082836e 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -89,6 +89,7 @@ int sysctl_max_map_count __read_mostly = DEFAULT_MAX_MAP_COUNT;
>    * other variables. It can be updated by several CPUs frequently.
>    */
>   struct percpu_counter vm_committed_as ____cacheline_aligned_in_smp;
> +int vm_committed_batchsz ____cacheline_aligned_in_smp;
>   
>   /*
>    * The global memory commitment made in the system can be a metric
> @@ -3090,10 +3091,20 @@ void mm_drop_all_locks(struct mm_struct *mm)
>   /*
>    * initialise the VMA slab
>    */
> +static inline int mm_compute_batch(void)
> +{
> +	int nr = num_present_cpus();
> +
> +	/* batch size set to 0.4% of (total memory/#cpus) */
> +	return (int) (totalram_pages/nr) / 256;
> +}
> +
>   void __init mmap_init(void)
>   {
>   	int ret;
>   
> -	ret = percpu_counter_init(&vm_committed_as, 0);
> +	vm_committed_batchsz = mm_compute_batch();
> +	ret = percpu_counter_and_batch_init(&vm_committed_as, 0,
> +						&vm_committed_batchsz);
>   	VM_BUG_ON(ret);
>   }
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 2f3ea74..a87a99c 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -59,6 +59,7 @@ unsigned long max_mapnr;
>   unsigned long num_physpages;
>   unsigned long highest_memmap_pfn;
>   struct percpu_counter vm_committed_as;
> +int vm_committed_batchsz;
>   int sysctl_overcommit_memory = OVERCOMMIT_GUESS; /* heuristic overcommit */
>   int sysctl_overcommit_ratio = 50; /* default is 50% */
>   int sysctl_max_map_count = DEFAULT_MAX_MAP_COUNT;
> @@ -526,11 +527,21 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
>   /*
>    * initialise the VMA and region record slabs
>    */
> +static inline int mm_compute_batch(void)
> +{
> +	int nr = num_present_cpus();
> +
> +	/* batch size set to 0.4% of (total memory/#cpus) */
> +	return (int) (totalram_pages/nr) / 256;
> +}
> +
>   void __init mmap_init(void)
>   {
>   	int ret;
>   
> -	ret = percpu_counter_init(&vm_committed_as, 0);
> +	vm_committed_batchsz = mm_compute_batch();
> +	ret = percpu_counter_and_batch_init(&vm_committed_as, 0,
> +			&vm_committed_batchsz);
>   	VM_BUG_ON(ret);
>   	vm_region_jar = KMEM_CACHE(vm_region, SLAB_PANIC);
>   }

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