[PATCH] eal: add cache guard to per-lcore PRNG state

[PATCH] eal: add cache guard to per-lcore PRNG state

[Morten Brørup]

The per-lcore random state is frequently updated by their individual
lcores, so add a cache guard to prevent CPU cache thrashing.

Depends-on: series-29415 ("clarify purpose of empty cache lines")

Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
---
 lib/eal/common/rte_random.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index 565f2401ce..3df0c7004a 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -18,6 +18,7 @@ struct rte_rand_state {
 	uint64_t z3;
 	uint64_t z4;
 	uint64_t z5;
+	RTE_CACHE_GUARD;
 } __rte_cache_aligned;
 
 /* One instance each for every lcore id-equipped thread, and one
-- 
2.17.1

Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Mattias Rönnblom]

On 2023-09-04 11:26, Morten Brørup wrote:
> The per-lcore random state is frequently updated by their individual
> lcores, so add a cache guard to prevent CPU cache thrashing.
> 

"to prevent false sharing in case the CPU employs a next-N-lines (or 
similar) hardware prefetcher"

In my world, cache trashing and cache line contention are two different 
things.

Other than that,
Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>

> Depends-on: series-29415 ("clarify purpose of empty cache lines")
> 
> Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> ---
>   lib/eal/common/rte_random.c | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> index 565f2401ce..3df0c7004a 100644
> --- a/lib/eal/common/rte_random.c
> +++ b/lib/eal/common/rte_random.c
> @@ -18,6 +18,7 @@ struct rte_rand_state {
>   	uint64_t z3;
>   	uint64_t z4;
>   	uint64_t z5;
> +	RTE_CACHE_GUARD;
>   } __rte_cache_aligned;
>   
>   /* One instance each for every lcore id-equipped thread, and one

Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Stephen Hemminger]

On Mon, 4 Sep 2023 13:57:19 +0200
Mattias Rönnblom <hofors@lysator.liu.se> wrote:

> On 2023-09-04 11:26, Morten Brørup wrote:
> > The per-lcore random state is frequently updated by their individual
> > lcores, so add a cache guard to prevent CPU cache thrashing.
> >   
> 
> "to prevent false sharing in case the CPU employs a next-N-lines (or 
> similar) hardware prefetcher"
> 
> In my world, cache trashing and cache line contention are two different 
> things.
> 
> Other than that,
> Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>

Could the per-lcore state be thread local?

Something like this:

From 3df5e28a7e5589d05e1eade62a0979e84697853d Mon Sep 17 00:00:00 2001
From: Stephen Hemminger <stephen@networkplumber.org>
Date: Wed, 6 Sep 2023 09:22:42 -0700
Subject: [PATCH] random: use per lcore state

Move the random number state into thread local storage.
This has a several benefits.
 - no false cache sharing from cpu prefetching
 - fixes initialization of random state for non-DPDK threads
 - fixes unsafe usage of random state by non-DPDK threads

The initialization of random number state is done by the
lcore (lazy initialization).

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/eal/common/rte_random.c | 35 +++++++++++++++++------------------
 1 file changed, 17 insertions(+), 18 deletions(-)

diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index 53636331a27b..62f36038ac52 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -19,13 +19,14 @@ struct rte_rand_state {
 	uint64_t z3;
 	uint64_t z4;
 	uint64_t z5;
-} __rte_cache_aligned;
+	uint64_t seed;
+};
 
-/* One instance each for every lcore id-equipped thread, and one
- * additional instance to be shared by all others threads (i.e., all
- * unregistered non-EAL threads).
- */
-static struct rte_rand_state rand_states[RTE_MAX_LCORE + 1];
+/* Global random seed */
+static uint64_t rte_rand_seed;
+
+/* Per lcore random state. */
+static RTE_DEFINE_PER_LCORE(struct rte_rand_state, rte_rand_state);
 
 static uint32_t
 __rte_rand_lcg32(uint32_t *seed)
@@ -76,16 +77,14 @@ __rte_srand_lfsr258(uint64_t seed, struct rte_rand_state *state)
 	state->z3 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 4096UL);
 	state->z4 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 131072UL);
 	state->z5 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 8388608UL);
+
+	state->seed = seed;
 }
 
 void
 rte_srand(uint64_t seed)
 {
-	unsigned int lcore_id;
-
-	/* add lcore_id to seed to avoid having the same sequence */
-	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++)
-		__rte_srand_lfsr258(seed + lcore_id, &rand_states[lcore_id]);
+	__atomic_store_n(&rte_rand_seed, seed, __ATOMIC_RELAXED);
 }
 
 static __rte_always_inline uint64_t
@@ -119,15 +118,15 @@ __rte_rand_lfsr258(struct rte_rand_state *state)
 static __rte_always_inline
 struct rte_rand_state *__rte_rand_get_state(void)
 {
-	unsigned int idx;
-
-	idx = rte_lcore_id();
+	struct rte_rand_state *rand_state = &RTE_PER_LCORE(rte_rand_state);
+	uint64_t seed;
 
-	/* last instance reserved for unregistered non-EAL threads */
-	if (unlikely(idx == LCORE_ID_ANY))
-		idx = RTE_MAX_LCORE;
+	/* did seed change */
+	seed = __atomic_load_n(&rte_rand_seed, __ATOMIC_RELAXED);
+	if (unlikely(seed != rand_state->seed))
+		__rte_srand_lfsr258(seed, rand_state);
 
-	return &rand_states[idx];
+	return rand_state;
 }
 
 uint64_t
-- 
2.39.2

RE: [PATCH] eal: add cache guard to per-lcore PRNG state

[Morten Brørup]

PING for review.

Stephen, the discussion took quite a few turns, but didn't seem to reach a better solution. If you don't object to this simple patch, could you please also ack/review it, so it can be applied.

> From: Mattias Rönnblom [mailto:hofors@lysator.liu.se]
> Sent: Monday, 4 September 2023 13.57
> 
> On 2023-09-04 11:26, Morten Brørup wrote:
> > The per-lcore random state is frequently updated by their individual
> > lcores, so add a cache guard to prevent CPU cache thrashing.
> >
> 
> "to prevent false sharing in case the CPU employs a next-N-lines (or
> similar) hardware prefetcher"
> 
> In my world, cache trashing and cache line contention are two different
> things.

You are right, Mattias.

I didn't think give the description much thought, and simply used "cache trashing" in a broad, general sense. I think most readers will get the point anyway. Or they could take a look at the description provided for the RTE_CACHE_GUARD itself. :-)

> 
> Other than that,
> Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> 
> > Depends-on: series-29415 ("clarify purpose of empty cache lines")
> >
> > Signed-off-by: Morten Brørup <mb@smartsharesystems.com>
> > ---
> >   lib/eal/common/rte_random.c | 1 +
> >   1 file changed, 1 insertion(+)
> >
> > diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> > index 565f2401ce..3df0c7004a 100644
> > --- a/lib/eal/common/rte_random.c
> > +++ b/lib/eal/common/rte_random.c
> > @@ -18,6 +18,7 @@ struct rte_rand_state {
> >   	uint64_t z3;
> >   	uint64_t z4;
> >   	uint64_t z5;
> > +	RTE_CACHE_GUARD;
> >   } __rte_cache_aligned;
> >
> >   /* One instance each for every lcore id-equipped thread, and one

Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Thomas Monjalon]

TLS is an alternative solution proposed by Stephen.
What do you think?


06/09/2023 18:25, Stephen Hemminger:
> On Mon, 4 Sep 2023 13:57:19 +0200
> Mattias Rönnblom <hofors@lysator.liu.se> wrote:
> 
> > On 2023-09-04 11:26, Morten Brørup wrote:
> > > The per-lcore random state is frequently updated by their individual
> > > lcores, so add a cache guard to prevent CPU cache thrashing.
> > >   
> > 
> > "to prevent false sharing in case the CPU employs a next-N-lines (or 
> > similar) hardware prefetcher"
> > 
> > In my world, cache trashing and cache line contention are two different 
> > things.
> > 
> > Other than that,
> > Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> 
> Could the per-lcore state be thread local?
> 
> Something like this:
> 
> From 3df5e28a7e5589d05e1eade62a0979e84697853d Mon Sep 17 00:00:00 2001
> From: Stephen Hemminger <stephen@networkplumber.org>
> Date: Wed, 6 Sep 2023 09:22:42 -0700
> Subject: [PATCH] random: use per lcore state
> 
> Move the random number state into thread local storage.
> This has a several benefits.
>  - no false cache sharing from cpu prefetching
>  - fixes initialization of random state for non-DPDK threads
>  - fixes unsafe usage of random state by non-DPDK threads
> 
> The initialization of random number state is done by the
> lcore (lazy initialization).
> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> ---
>  lib/eal/common/rte_random.c | 35 +++++++++++++++++------------------
>  1 file changed, 17 insertions(+), 18 deletions(-)
> 
> diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> index 53636331a27b..62f36038ac52 100644
> --- a/lib/eal/common/rte_random.c
> +++ b/lib/eal/common/rte_random.c
> @@ -19,13 +19,14 @@ struct rte_rand_state {
>  	uint64_t z3;
>  	uint64_t z4;
>  	uint64_t z5;
> -} __rte_cache_aligned;
> +	uint64_t seed;
> +};
>  
> -/* One instance each for every lcore id-equipped thread, and one
> - * additional instance to be shared by all others threads (i.e., all
> - * unregistered non-EAL threads).
> - */
> -static struct rte_rand_state rand_states[RTE_MAX_LCORE + 1];
> +/* Global random seed */
> +static uint64_t rte_rand_seed;
> +
> +/* Per lcore random state. */
> +static RTE_DEFINE_PER_LCORE(struct rte_rand_state, rte_rand_state);
>  
>  static uint32_t
>  __rte_rand_lcg32(uint32_t *seed)
> @@ -76,16 +77,14 @@ __rte_srand_lfsr258(uint64_t seed, struct rte_rand_state *state)
>  	state->z3 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 4096UL);
>  	state->z4 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 131072UL);
>  	state->z5 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 8388608UL);
> +
> +	state->seed = seed;
>  }
>  
>  void
>  rte_srand(uint64_t seed)
>  {
> -	unsigned int lcore_id;
> -
> -	/* add lcore_id to seed to avoid having the same sequence */
> -	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++)
> -		__rte_srand_lfsr258(seed + lcore_id, &rand_states[lcore_id]);
> +	__atomic_store_n(&rte_rand_seed, seed, __ATOMIC_RELAXED);
>  }
>  
>  static __rte_always_inline uint64_t
> @@ -119,15 +118,15 @@ __rte_rand_lfsr258(struct rte_rand_state *state)
>  static __rte_always_inline
>  struct rte_rand_state *__rte_rand_get_state(void)
>  {
> -	unsigned int idx;
> -
> -	idx = rte_lcore_id();
> +	struct rte_rand_state *rand_state = &RTE_PER_LCORE(rte_rand_state);
> +	uint64_t seed;
>  
> -	/* last instance reserved for unregistered non-EAL threads */
> -	if (unlikely(idx == LCORE_ID_ANY))
> -		idx = RTE_MAX_LCORE;
> +	/* did seed change */
> +	seed = __atomic_load_n(&rte_rand_seed, __ATOMIC_RELAXED);
> +	if (unlikely(seed != rand_state->seed))
> +		__rte_srand_lfsr258(seed, rand_state);
>  
> -	return &rand_states[idx];
> +	return rand_state;
>  }
>  
>  uint64_t
> 

RE: [PATCH] eal: add cache guard to per-lcore PRNG state

[Morten Brørup]

> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Wednesday, 11 October 2023 18.08
> 
> TLS is an alternative solution proposed by Stephen.
> What do you think?

I think we went down a rabbit hole - which I admit to enjoy. :-)

My simple patch should be applied, with the description improved by Mattias:

The per-lcore random state is frequently updated by their individual
lcores, so add a cache guard to prevent false sharing in case the
CPU employs a next-N-lines (or similar) hardware prefetcher.

> 
> 
> 06/09/2023 18:25, Stephen Hemminger:
> > On Mon, 4 Sep 2023 13:57:19 +0200
> > Mattias Rönnblom <hofors@lysator.liu.se> wrote:
> >
> > > On 2023-09-04 11:26, Morten Brørup wrote:
> > > > The per-lcore random state is frequently updated by their
> individual
> > > > lcores, so add a cache guard to prevent CPU cache thrashing.
> > > >
> > >
> > > "to prevent false sharing in case the CPU employs a next-N-lines (or
> > > similar) hardware prefetcher"
> > >
> > > In my world, cache trashing and cache line contention are two
> different
> > > things.
> > >
> > > Other than that,
> > > Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
> >
> > Could the per-lcore state be thread local?
> >
> > Something like this:
> >
> > From 3df5e28a7e5589d05e1eade62a0979e84697853d Mon Sep 17 00:00:00 2001
> > From: Stephen Hemminger <stephen@networkplumber.org>
> > Date: Wed, 6 Sep 2023 09:22:42 -0700
> > Subject: [PATCH] random: use per lcore state
> >
> > Move the random number state into thread local storage.
> > This has a several benefits.
> >  - no false cache sharing from cpu prefetching
> >  - fixes initialization of random state for non-DPDK threads
> >  - fixes unsafe usage of random state by non-DPDK threads
> >
> > The initialization of random number state is done by the
> > lcore (lazy initialization).
> >
> > Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> > ---
> >  lib/eal/common/rte_random.c | 35 +++++++++++++++++------------------
> >  1 file changed, 17 insertions(+), 18 deletions(-)
> >
> > diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
> > index 53636331a27b..62f36038ac52 100644
> > --- a/lib/eal/common/rte_random.c
> > +++ b/lib/eal/common/rte_random.c
> > @@ -19,13 +19,14 @@ struct rte_rand_state {
> >  	uint64_t z3;
> >  	uint64_t z4;
> >  	uint64_t z5;
> > -} __rte_cache_aligned;
> > +	uint64_t seed;
> > +};
> >
> > -/* One instance each for every lcore id-equipped thread, and one
> > - * additional instance to be shared by all others threads (i.e., all
> > - * unregistered non-EAL threads).
> > - */
> > -static struct rte_rand_state rand_states[RTE_MAX_LCORE + 1];
> > +/* Global random seed */
> > +static uint64_t rte_rand_seed;
> > +
> > +/* Per lcore random state. */
> > +static RTE_DEFINE_PER_LCORE(struct rte_rand_state, rte_rand_state);
> >
> >  static uint32_t
> >  __rte_rand_lcg32(uint32_t *seed)
> > @@ -76,16 +77,14 @@ __rte_srand_lfsr258(uint64_t seed, struct
> rte_rand_state *state)
> >  	state->z3 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 4096UL);
> >  	state->z4 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 131072UL);
> >  	state->z5 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 8388608UL);
> > +
> > +	state->seed = seed;
> >  }
> >
> >  void
> >  rte_srand(uint64_t seed)
> >  {
> > -	unsigned int lcore_id;
> > -
> > -	/* add lcore_id to seed to avoid having the same sequence */
> > -	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++)
> > -		__rte_srand_lfsr258(seed + lcore_id,
> &rand_states[lcore_id]);
> > +	__atomic_store_n(&rte_rand_seed, seed, __ATOMIC_RELAXED);
> >  }
> >
> >  static __rte_always_inline uint64_t
> > @@ -119,15 +118,15 @@ __rte_rand_lfsr258(struct rte_rand_state *state)
> >  static __rte_always_inline
> >  struct rte_rand_state *__rte_rand_get_state(void)
> >  {
> > -	unsigned int idx;
> > -
> > -	idx = rte_lcore_id();
> > +	struct rte_rand_state *rand_state =
> &RTE_PER_LCORE(rte_rand_state);
> > +	uint64_t seed;
> >
> > -	/* last instance reserved for unregistered non-EAL threads */
> > -	if (unlikely(idx == LCORE_ID_ANY))
> > -		idx = RTE_MAX_LCORE;
> > +	/* did seed change */
> > +	seed = __atomic_load_n(&rte_rand_seed, __ATOMIC_RELAXED);
> > +	if (unlikely(seed != rand_state->seed))
> > +		__rte_srand_lfsr258(seed, rand_state);
> >
> > -	return &rand_states[idx];
> > +	return rand_state;
> >  }
> >
> >  uint64_t
> >
> 
> 
> 
> 

Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Mattias Rönnblom]

On 2023-10-11 18:07, Thomas Monjalon wrote:
> TLS is an alternative solution proposed by Stephen.
> What do you think?
> 

I've expressed my views on this topic in two threads already.

I'm happy to continue that discussion, but I would suggest it would be 
under the banner of "what should the standard pattern for maintaining 
per-lcore (and maybe also per-unregistered thread state) be in DPDK".

A related issue is the ambition level for having unregistered threads 
calling into DPDK APIs in general. MT safety issues, performance issues, 
and concerns around preemption safety.

> 
> 06/09/2023 18:25, Stephen Hemminger:
>> On Mon, 4 Sep 2023 13:57:19 +0200
>> Mattias Rönnblom <hofors@lysator.liu.se> wrote:
>>
>>> On 2023-09-04 11:26, Morten Brørup wrote:
>>>> The per-lcore random state is frequently updated by their individual
>>>> lcores, so add a cache guard to prevent CPU cache thrashing.
>>>>    
>>>
>>> "to prevent false sharing in case the CPU employs a next-N-lines (or
>>> similar) hardware prefetcher"
>>>
>>> In my world, cache trashing and cache line contention are two different
>>> things.
>>>
>>> Other than that,
>>> Acked-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>
>> Could the per-lcore state be thread local?
>>
>> Something like this:
>>
>>  From 3df5e28a7e5589d05e1eade62a0979e84697853d Mon Sep 17 00:00:00 2001
>> From: Stephen Hemminger <stephen@networkplumber.org>
>> Date: Wed, 6 Sep 2023 09:22:42 -0700
>> Subject: [PATCH] random: use per lcore state
>>
>> Move the random number state into thread local storage.
>> This has a several benefits.
>>   - no false cache sharing from cpu prefetching
>>   - fixes initialization of random state for non-DPDK threads
>>   - fixes unsafe usage of random state by non-DPDK threads
>>
>> The initialization of random number state is done by the
>> lcore (lazy initialization).
>>
>> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
>> ---
>>   lib/eal/common/rte_random.c | 35 +++++++++++++++++------------------
>>   1 file changed, 17 insertions(+), 18 deletions(-)
>>
>> diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
>> index 53636331a27b..62f36038ac52 100644
>> --- a/lib/eal/common/rte_random.c
>> +++ b/lib/eal/common/rte_random.c
>> @@ -19,13 +19,14 @@ struct rte_rand_state {
>>   	uint64_t z3;
>>   	uint64_t z4;
>>   	uint64_t z5;
>> -} __rte_cache_aligned;
>> +	uint64_t seed;
>> +};
>>   
>> -/* One instance each for every lcore id-equipped thread, and one
>> - * additional instance to be shared by all others threads (i.e., all
>> - * unregistered non-EAL threads).
>> - */
>> -static struct rte_rand_state rand_states[RTE_MAX_LCORE + 1];
>> +/* Global random seed */
>> +static uint64_t rte_rand_seed;
>> +
>> +/* Per lcore random state. */
>> +static RTE_DEFINE_PER_LCORE(struct rte_rand_state, rte_rand_state);
>>   
>>   static uint32_t
>>   __rte_rand_lcg32(uint32_t *seed)
>> @@ -76,16 +77,14 @@ __rte_srand_lfsr258(uint64_t seed, struct rte_rand_state *state)
>>   	state->z3 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 4096UL);
>>   	state->z4 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 131072UL);
>>   	state->z5 = __rte_rand_lfsr258_gen_seed(&lcg_seed, 8388608UL);
>> +
>> +	state->seed = seed;
>>   }
>>   
>>   void
>>   rte_srand(uint64_t seed)
>>   {
>> -	unsigned int lcore_id;
>> -
>> -	/* add lcore_id to seed to avoid having the same sequence */
>> -	for (lcore_id = 0; lcore_id < RTE_MAX_LCORE; lcore_id++)
>> -		__rte_srand_lfsr258(seed + lcore_id, &rand_states[lcore_id]);
>> +	__atomic_store_n(&rte_rand_seed, seed, __ATOMIC_RELAXED);
>>   }
>>   
>>   static __rte_always_inline uint64_t
>> @@ -119,15 +118,15 @@ __rte_rand_lfsr258(struct rte_rand_state *state)
>>   static __rte_always_inline
>>   struct rte_rand_state *__rte_rand_get_state(void)
>>   {
>> -	unsigned int idx;
>> -
>> -	idx = rte_lcore_id();
>> +	struct rte_rand_state *rand_state = &RTE_PER_LCORE(rte_rand_state);
>> +	uint64_t seed;
>>   
>> -	/* last instance reserved for unregistered non-EAL threads */
>> -	if (unlikely(idx == LCORE_ID_ANY))
>> -		idx = RTE_MAX_LCORE;
>> +	/* did seed change */
>> +	seed = __atomic_load_n(&rte_rand_seed, __ATOMIC_RELAXED);
>> +	if (unlikely(seed != rand_state->seed))
>> +		__rte_srand_lfsr258(seed, rand_state);
>>   
>> -	return &rand_states[idx];
>> +	return rand_state;
>>   }
>>   
>>   uint64_t
>>
> 
> 
> 
> 
> 

Re: [PATCH] eal: add cache guard to per-lcore PRNG state

[Thomas Monjalon]

11/10/2023 18:55, Morten Brørup:
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > Sent: Wednesday, 11 October 2023 18.08
> > 
> > TLS is an alternative solution proposed by Stephen.
> > What do you think?
> 
> I think we went down a rabbit hole - which I admit to enjoy. :-)

There is no reply/explanation in this thread.

> My simple patch should be applied, with the description improved by Mattias:
> 
> The per-lcore random state is frequently updated by their individual
> lcores, so add a cache guard to prevent false sharing in case the
> CPU employs a next-N-lines (or similar) hardware prefetcher.

OK applied