[ovs-dev] [PATCH v3 1/2] dpif-netdev: Decrease range of values for EMC probability.
Wang, Yipeng1
yipeng1.wang at intel.com
Fri Aug 4 20:39:10 UTC 2017
> -----Original Message-----
> From: Darrell Ball [mailto:dball at vmware.com]
> Sent: Friday, August 4, 2017 11:35 AM
> To: Ilya Maximets <i.maximets at samsung.com>; ovs-dev at openvswitch.org
> Cc: Heetae Ahn <heetae82.ahn at samsung.com>; Wang, Yipeng1
> <yipeng1.wang at intel.com>; Kevin Traynor <ktraynor at redhat.com>; Loftus,
> Ciara <ciara.loftus at intel.com>; Fischetti, Antonio
> <antonio.fischetti at intel.com>
> Subject: Re: [PATCH v3 1/2] dpif-netdev: Decrease range of values for EMC
> probability.
>
>
>
> -----Original Message-----
> From: Ilya Maximets <i.maximets at samsung.com>
> Date: Friday, August 4, 2017 at 7:17 AM
> To: "ovs-dev at openvswitch.org" <ovs-dev at openvswitch.org>
> Cc: Heetae Ahn <heetae82.ahn at samsung.com>, Darrell Ball
> <dball at vmware.com>, Yipeng Wang <yipeng1.wang at intel.com>, Kevin
> Traynor <ktraynor at redhat.com>, Ciara Loftus <ciara.loftus at intel.com>,
> Antonio Fischetti <antonio.fischetti at intel.com>, Ilya Maximets
> <i.maximets at samsung.com>
> Subject: [PATCH v3 1/2] dpif-netdev: Decrease range of values for EMC
> probability.
>
> Currently, real insertion probability is higher than configured
> for the maximum case because of wrong usage of the random value.
>
> i.e. if 'emc-invert-inv-prob' == UINT32_MAX, then 'emc_insert_min'
> equals to 1. In this case we're allowing insert if random vailue
> is less or equal to 1. So, two of 2**32 values (0 and 1) are
> allowed and real probability is 2 times higher than configured.
>
> This happens because 'random_uint32()' returns value in range
> [0; UINT32_MAX], but for the checking to be correct we should
> generate random value in range [0; UINT32_MAX - 1].
>
>
> I understand the calculation is slightly off.
> If the user enters 4,294,967,295 then the probability to insert into emc will be
> 2 out of 4,294,967,295 rather than 1 out of 4,294,967,295,
>
> However, is there a general concern about such a low probability anyways ?
> This max inverse value would be rarely, if ever used and if used, it would be
> impossible
> to see the difference in any real use case.
>
> The user might as well just disable emc rather than use such tiny probabilities.
>
> This existing api was discussed extensively and was very contentious.
>
> However, if patch 2 really has an absolute dependency on this patch 1, we
> can include it.
> I have done various testing and don’t see that, but I have some comments
> on the
> other threads.
>
>
[Wang, Yipeng] The dependency I can tell is that when do EMC_insert, the random bit is chosen by (random_value >> EM_FLOW_INSERT_INV_PROB_SHIFT & 1) in next patch, which means that the random bit will be "fully independent".
If it is the only dependency, I guess it is fine to even not include this patch.
> To fix this we have 4 possible solutions:
>
> 1. need to use uint64_t for 'emc-insert-min' and calculate it
> as '(UINT32_MAX + 1) / inverse_prob' to fairly check the full
> range [0; UINT32_MAX].
>
> This may decrease performance becaue of 64 bit atomic ops.
>
> 2. Forbid the '2**32 - 1' as the value for 'emc-insert-min'
> because it's the only value we have issues with.
>
> This will require additional explanations and not very friendly
> for users.
>
> 3. Generate random value in range [0; UINT32_MAX - 1].
>
> This will require heavy division operation.
>
> 4. Decrease the range of available values for 'emc-insert-inv-prob'.
>
> Actually, we don't need to have so much different values for
> that option. I beleve that values higher than 1M are completely
> useless. Choosing the upper limit as a power of 2 like 2**20 we
> will be able to mask the generated random value in a fast way
> and also avoid range issue, because same uint32_t can be used to
> store 2**20.
>
> This patch implements solution #4.
>
> CC: Ciara Loftus <ciara.loftus at intel.com>
> Fixes: 4c30b24602c3 ("dpif-netdev: Conditional EMC insert")
> Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
> Acked-by: Antonio Fischetti <antonio.fischetti at intel.com>
> ---
>
> Infrastructure and logic introduced here will be used for fixing
> emc replacement policy.
>
> lib/dpif-netdev.c | 14 ++++++++++----
> vswitchd/vswitch.xml | 3 ++-
> 2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 8ecc9d1..e23c674 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -153,9 +153,14 @@ struct netdev_flow_key {
> #define EM_FLOW_HASH_MASK (EM_FLOW_HASH_ENTRIES - 1)
> #define EM_FLOW_HASH_SEGS 2
>
> +/* Set up maximum inverse EMC insertion probability to 2^20 - 1.
> + * Higher values considered useless in practice. */
> +#define EM_FLOW_INSERT_INV_PROB_SHIFT 20
> +#define EM_FLOW_INSERT_INV_PROB_MAX (1 <<
> EM_FLOW_INSERT_INV_PROB_SHIFT)
> +#define EM_FLOW_INSERT_INV_PROB_MASK
> (EM_FLOW_INSERT_INV_PROB_MAX - 1)
> /* Default EMC insert probability is 1 /
> DEFAULT_EM_FLOW_INSERT_INV_PROB */
> #define DEFAULT_EM_FLOW_INSERT_INV_PROB 100
> -#define DEFAULT_EM_FLOW_INSERT_MIN (UINT32_MAX / \
> +#define DEFAULT_EM_FLOW_INSERT_MIN
> (EM_FLOW_INSERT_INV_PROB_MAX / \
> DEFAULT_EM_FLOW_INSERT_INV_PROB)
>
> struct emc_entry {
> @@ -2092,7 +2097,7 @@ emc_probabilistic_insert(struct
> dp_netdev_pmd_thread *pmd,
> uint32_t min;
> atomic_read_relaxed(&pmd->dp->emc_insert_min, &min);
>
> - if (min && random_uint32() <= min) {
> + if (min && (random_uint32() & EM_FLOW_INSERT_INV_PROB_MASK)
> < min) {
> emc_insert(&pmd->flow_cache, key, flow);
> }
> }
> @@ -2909,8 +2914,9 @@ dpif_netdev_set_config(struct dpif *dpif, const
> struct smap *other_config)
> }
>
> atomic_read_relaxed(&dp->emc_insert_min, &cur_min);
> - if (insert_prob <= UINT32_MAX) {
> - insert_min = insert_prob == 0 ? 0 : UINT32_MAX / insert_prob;
> + if (insert_prob < EM_FLOW_INSERT_INV_PROB_MAX) {
> + insert_min = insert_prob == 0
> + ? 0 : EM_FLOW_INSERT_INV_PROB_MAX / insert_prob;
> } else {
> insert_min = DEFAULT_EM_FLOW_INSERT_MIN;
> insert_prob = DEFAULT_EM_FLOW_INSERT_INV_PROB;
> diff --git a/vswitchd/vswitch.xml b/vswitchd/vswitch.xml
> index 074535b..61f252e 100644
> --- a/vswitchd/vswitch.xml
> +++ b/vswitchd/vswitch.xml
> @@ -381,7 +381,8 @@
> </column>
>
> <column name="other_config" key="emc-insert-inv-prob"
> - type='{"type": "integer", "minInteger": 0, "maxInteger":
> 4294967295}'>
> + type='{"type": "integer",
> + "minInteger": 0, "maxInteger": 1048575}'>
> <p>
> Specifies the inverse probability (1/emc-insert-inv-prob) of a flow
> being inserted into the Exact Match Cache (EMC). On average one in
> --
> 2.7.4
>
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