[ovs-dev] [PATCH] dpif-netdev: Simplify emc replacement policy.
Ilya Maximets
i.maximets at samsung.com
Mon Jul 31 14:25:20 UTC 2017
On 31.07.2017 04:41, Darrell Ball wrote:
>
>
> -----Original Message-----
> From: <ovs-dev-bounces at openvswitch.org> on behalf of "Wang, Yipeng1" <yipeng1.wang at intel.com>
> Date: Friday, July 28, 2017 at 11:04 AM
> To: Ilya Maximets <i.maximets at samsung.com>, "ovs-dev at openvswitch.org" <ovs-dev at openvswitch.org>
> Cc: Heetae Ahn <heetae82.ahn at samsung.com>
> Subject: Re: [ovs-dev] [PATCH] dpif-netdev: Simplify emc replacement policy.
>
> Good catch. But I think the hash comparison is to "randomly" choose one of the two entries to replace when both entries are live.
> Your change would always replace the first one in such case. It might cause some thrashing issue for certain traffic. Meanwhile, to my experience, the original "hash comparison" is also not a good way to choose random entry, I encountered some thrashing issue before.
>
> I think we want some condition like below, but a way to fast choose a random entry.
>
> if (!to_be_replaced || (emc_entry_alive(to_be_replaced) && !emc_entry_alive(current_entry) )
> to_be_replaced = current_entry;
> else if((emc_entry_alive(to_be_replaced) && (emc_entry_alive(current_entry))
> to_be_replaced = random_entry;
I agree that we need to have something like random choosing of active entry to replace.
I though about this a little and came up with idea to reuse the random value generated
for insertion probability. This should give a good distribution for replacement.
I'll send v2 soon with that approach.
> //////////////////
>
> I agree – we are trying to randomly select one of two live entries with the last condition.
> Something like this maybe makes it more clear what we are trying to do ?
Your code solves the issue with replacement of alive entries while dead ones exists,
but you're still uses hashes as random values which is not right. Hashes are not random
and there is no any difference in choosing the first entry or the entry with a bit
set in a particular place. There always will be some bad case where you will replace
same entries all the time and performance of EMC will be low.
> diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> index 47a9fa0..75cc039 100644
> --- a/lib/dpif-netdev.c
> +++ b/lib/dpif-netdev.c
> @@ -2051,12 +2051,15 @@ emc_insert(struct emc_cache *cache, const struct netdev_flow_key *key,
> }
>
> /* Replacement policy: put the flow in an empty (not alive) entry, or
> - * in the first entry where it can be */
> - if (!to_be_replaced
> - || (emc_entry_alive(to_be_replaced)
> - && !emc_entry_alive(current_entry))
> - || current_entry->key.hash < to_be_replaced->key.hash) {
> + * randomly choose one of the two alive entries to be replaced. */
> + if (!to_be_replaced) {
> to_be_replaced = current_entry;
> + } else if (emc_entry_alive(to_be_replaced) && !emc_entry_alive(current_entry)) {
> + to_be_replaced = current_entry;
> + } else if (emc_entry_alive(to_be_replaced) && emc_entry_alive(current_entry)) {
> + if (current_entry->key.hash & 0x1) {
> + to_be_replaced = current_entry;
> + }
> }
> }
> /* We didn't find the miniflow in the cache.
>
> //////////////////
>
> Thanks
> Yipeng
>
> > -----Original Message-----
> > From: ovs-dev-bounces at openvswitch.org [mailto:ovs-dev-
> > bounces at openvswitch.org] On Behalf Of Ilya Maximets
> > Sent: Friday, July 28, 2017 5:41 AM
> > To: ovs-dev at openvswitch.org
> > Cc: Ilya Maximets <i.maximets at samsung.com>; Heetae Ahn
> > <heetae82.ahn at samsung.com>
> > Subject: [ovs-dev] [PATCH] dpif-netdev: Simplify emc replacement policy.
> >
> > Current EMC replacement policy allows to replace active EMC entry
> > even if there are dead (empty) entries available. This leads to
> > EMC trashing even on few hundreds of flows. In some cases PMD
> > threads starts to execute classifier lookups even in tests with
> > 50 - 100 active flows.
> >
> > Fix this by removing of srtange hash comparison rule from the
> > replacement checking. New behavior also matches the comment that
> > describes replacement policy. This comment wasn't correct before.
> >
> > Testing shows stable work of exact match cache without misses
> > with up to 3072 active flows and only 0.05% of EMC misses with
> > 4096 flows. With higher number of flows there is no significant
> > difference with current implementation.
> >
> > For the reference, number of EMC misses in current master is
> > around 20% for the case with 2048 active flows.
> >
> > Testing performed with 100% EMC insert probability.
> >
> > Signed-off-by: Ilya Maximets <i.maximets at samsung.com>
> > ---
> > lib/dpif-netdev.c | 3 +--
> > 1 file changed, 1 insertion(+), 2 deletions(-)
> >
> > diff --git a/lib/dpif-netdev.c b/lib/dpif-netdev.c
> > index 47a9fa0..4a8dd80 100644
> > --- a/lib/dpif-netdev.c
> > +++ b/lib/dpif-netdev.c
> > @@ -2054,8 +2054,7 @@ emc_insert(struct emc_cache *cache, const struct
> > netdev_flow_key *key,
> > * in the first entry where it can be */
> > if (!to_be_replaced
> > || (emc_entry_alive(to_be_replaced)
> > - && !emc_entry_alive(current_entry))
> > - || current_entry->key.hash < to_be_replaced->key.hash) {
> > + && !emc_entry_alive(current_entry))) {
> > to_be_replaced = current_entry;
> > }
> > }
> > --
> > 2.7.4
> >
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