Re: [dpdk-dev] [PATCH] net/mlx5: introduce mlx5 hash list

[Slava Ovsiienko <viacheslavo@mellanox.com>]

> -----Original Message-----
> From: Stephen Hemminger <stephen@networkplumber.org>
> Sent: Tuesday, November 5, 2019 19:21
> To: Bing Zhao <bingz@mellanox.com>
> Cc: Ori Kam <orika@mellanox.com>; Slava Ovsiienko
> <viacheslavo@mellanox.com>; Raslan Darawsheh
> <rasland@mellanox.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] net/mlx5: introduce mlx5 hash list
> 
> On Tue,  5 Nov 2019 17:28:00 +0200
> Bing Zhao <bingz@mellanox.com> wrote:
> 
> > Introduce simple hash list to the mlx5 utilities. User can define its
> > own data structure containing the mlx5_hlist_entry and create the hash
> > list table via the creation interface. Then the entry will be inserted
> > into the table and linked to the corresponding list head. User should
> > guarantee there is no collision of the key and provide a callback
> > function to handle all the remaining entries in the table when
> > destroying the hash list. User should define a proper number of the
> > list heads in the table in order to get a better performance. The LSB
> > of the 'key' is used to calculate the index of the head in the list
> > heads array.
> > This implementation is not multi-threads safe right now.
> >
> > Signed-off-by: Bing Zhao <bingz@mellanox.com>
> 
> There is already a plethora of hash routines in DPDK, why introduce a
> chained hash list like Linux kernel?

We have multiple features in mlx5 requiring the run-time extendable storage.
For example, application/user might not know how many Flows are going to come
into play, it is not nice to allocate gigabytes at startup, assuming some fixed upper limit (10M+).
Simple lists (used before) are too slow, update the DPDK Cuckoo to be scaled is not trivial
and requires some time, so - simple hashed list seems  to be acceptable palliative for now.

With best regards, Slava