LGTM, just wondering if a further comment would help, eg something like +/* random_uint32() returns a value in the [0; UINT32_MAX] range. + For our checking to be correct we would need instead a random value + in the range [0; UINT32_MAX - 1]. To avoid further computation + we use a decreased range of available values for 'emc-insert-inv-prob' + ie [0; 2**20 - 1]. */ +#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)
? -Antonio > -----Original Message----- > From: ovs-dev-boun...@openvswitch.org [mailto:ovs-dev-boun...@openvswitch.org] > On Behalf Of Ilya Maximets > Sent: Monday, July 31, 2017 3:41 PM > To: ovs-dev@openvswitch.org > Cc: Heetae Ahn <heetae82....@samsung.com>; Ilya Maximets > <i.maxim...@samsung.com> > Subject: [ovs-dev] [PATCH v2 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]. > > 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.lof...@intel.com> > Fixes: 4c30b24602c3 ("dpif-netdev: Conditional EMC insert") > Signed-off-by: Ilya Maximets <i.maxim...@samsung.com> > --- > > Infrastructure and logic introduced here will be used for fixing > emc replacement policy. > > lib/dpif-netdev.c | 12 ++++++++---- > vswitchd/vswitch.xml | 3 ++- > 2 files changed, 10 insertions(+), 5 deletions(-) > > diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c > index 47a9fa0..123a7c9 100644 > --- a/lib/dpif-netdev.c > +++ b/lib/dpif-netdev.c > @@ -152,9 +152,12 @@ struct netdev_flow_key { > #define EM_FLOW_HASH_MASK (EM_FLOW_HASH_ENTRIES - 1) > #define EM_FLOW_HASH_SEGS 2 > > +#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 { > @@ -2077,7 +2080,7 @@ 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); > } > } > @@ -2894,8 +2897,9 @@ 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 > > _______________________________________________ > dev mailing list > d...@openvswitch.org > https://mail.openvswitch.org/mailman/listinfo/ovs-dev _______________________________________________ dev mailing list d...@openvswitch.org https://mail.openvswitch.org/mailman/listinfo/ovs-dev
