[ovs-dev] [PATCH v3 1/2] dpif-netdev: Decrease range of values for EMC probability.

Ilya Maximets i.maximets at samsung.com
Mon Aug 7 11:54:21 UTC 2017


On 04.08.2017 23:39, Wang, Yipeng1 wrote:
> 
> 
>> -----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".

Yes. The dependency is that bits of random value that passes (random_value < min) are
not fully random. Using them we may have additional issues with random slot choosing.
Even if we'll have large enough 'min' value we may have worse distribution of lower
bits than with original PRNG implemented in lib/random.

> 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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