Re: [Xen-devel] [PATCH 1 of 2 RFC] xen, pod: Zero-check recently populated pages (checklast)

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.06.12 at 13:45, George Dunlap <george.dunlap@xxxxxxxxxxxxx> wrote:
> When demand-populating pages due to guest accesses, check recently populated
> pages to see if we can reclaim them for the cache.  This should keep the PoD
> cache filled when the start-of-day scrubber is going through.
> 
> The number 128 was chosen by experiment.  Windows does its page
> scrubbing in parallel; while a small nubmer like 4 works well for
> single VMs, it breaks down as multiple vcpus are scrubbing different
> pages in parallel.  Increasing to 128 works well for higher numbers of
> vcpus.
> 
> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxxxxx>
> 
> diff --git a/xen/arch/x86/mm/p2m-pod.c b/xen/arch/x86/mm/p2m-pod.c
> --- a/xen/arch/x86/mm/p2m-pod.c
> +++ b/xen/arch/x86/mm/p2m-pod.c
> @@ -909,6 +909,26 @@ p2m_pod_emergency_sweep_super(struct p2m
>      p2m->pod.reclaim_super = i ? i - SUPERPAGE_PAGES : 0;
>  }
>  
> +/* When populating a new superpage, look at recently populated superpages
> + * hoping that they've been zeroed.  This will snap up zeroed pages as soon 
> as 
> + * the guest OS is done with them. */
> +static void
> +p2m_pod_check_last_super(struct p2m_domain *p2m, unsigned long gfn_aligned)
> +{
> +    unsigned long check_gfn;
> +
> +    ASSERT(p2m->pod.last_populated_index < POD_HISTORY_MAX);
> +
> +    check_gfn = p2m->pod.last_populated[p2m->pod.last_populated_index];
> +
> +    p2m->pod.last_populated[p2m->pod.last_populated_index] = gfn_aligned;
> +
> +    p2m->pod.last_populated_index = ( p2m->pod.last_populated_index + 1 ) % 
> POD_HISTORY_MAX;
> +
> +    p2m->pod.reclaim_super += p2m_pod_zero_check_superpage(p2m, check_gfn);
> +}
> +
> +
>  #define POD_SWEEP_STRIDE  16
>  static void
>  p2m_pod_emergency_sweep(struct p2m_domain *p2m)
> @@ -1066,6 +1086,12 @@ p2m_pod_demand_populate(struct p2m_domai
>          __trace_var(TRC_MEM_POD_POPULATE, 0, sizeof(t), &t);
>      }
>  
> +    /* Check the last guest demand-populate */
> +    if ( p2m->pod.entry_count > p2m->pod.count 
> +         && order == 9
> +         && q & P2M_ALLOC )
> +        p2m_pod_check_last_super(p2m, gfn_aligned);
> +
>      pod_unlock(p2m);
>      return 0;
>  out_of_memory:
> diff --git a/xen/include/asm-x86/p2m.h b/xen/include/asm-x86/p2m.h
> --- a/xen/include/asm-x86/p2m.h
> +++ b/xen/include/asm-x86/p2m.h
> @@ -287,6 +287,9 @@ struct p2m_domain {
>          unsigned         reclaim_super; /* Last gpfn of a scan */
>          unsigned         reclaim_single; /* Last gpfn of a scan */
>          unsigned         max_guest;    /* gpfn of max guest demand-populate 
> */
> +#define POD_HISTORY_MAX 128
> +        unsigned         last_populated[POD_HISTORY_MAX]; /* gpfn of last 
> guest page demand-populated */

unsigned long?

Also, wouldn't it be better to allocate this table dynamically, at
once allowing its size to scale with the number of vCPU-s in the
guest?

> +        int              last_populated_index;

'unsigned int' is generally better suited for array indexes (and
definitely on x86-64).

Jan

>          mm_lock_t        lock;         /* Locking of private pod structs,   
> *
>                                          * not relying on the p2m lock.      
> */
>      } pod;
> 
> _______________________________________________
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> Xen-devel@xxxxxxxxxxxxx 
> http://lists.xen.org/xen-devel 




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