Subject: Re: [PATCH RFC net-next] net: Poptrie based FIB lookup

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 27 Aug 2018 17:33:34 +0200

On Sun, 26 Aug 2018 22:13:53 -0400 "Md. Islam" <[email-masked]> wrote:

> This patch implements Poptrie [1] based FIB lookup. It exhibits pretty
> impressive lookup performance compared to LC-trie. This poptrie
> implementation however somewhat deviates from the original
> implementation [2]. I tested this patch very rigorously with several
> FIB tables containing half a million routes. I got same result as
> LC-trie based fib_lookup().

It sounds really promising performance wise. The article [1] claim
lookup speeds up-to 240 Million lookups per second.  That is a crazy
speed. This is 4.166 nanosec per lookup (1/240*1000), and their test
CPU is 3.9GHz, which gives them 16.25 CPU cycles for a lookup, with 3
insn per cycle, that gives them max 48 perfectly pipelined instructions
per lookup.

> Poptrie is intended to work in conjunction with LC-trie (not replace
> it). It is primarily designed to overcome many issues of TCAM based
> router [1]. [1] shows that the Poptrie can achieve very impressive
> lookup performance on CPU. This patch will mainly be used by XDP
> forwarding.
> 
> 1. Asai, Hirochika, and Yasuhiro Ohara. "Poptrie: A compressed trie
> with population count for fast and scalable software IP routing table
> lookup." ACM SIGCOMM Computer Communication Review. 2015.
> 
> 2. https://github.com/pixos/poptrie
> 
> From c5e05ea66b06eb9313749bc8969b4c2798fcf96a Mon Sep 17 00:00:00 2001
> From: tamimcse <[email-masked]>
> Date: Sun, 26 Aug 2018 21:12:38 -0400
> Subject: [PATCH] Implented Poptrie

This above "commit-info" should not be part of the patch description.

> Signed-off-by: tamimcse <[email-masked]>

Use you real/full name here.

> ---

First of order of business: You need to conform to the kernels coding
standards!

https://www.kernel.org/doc/html/v4.18/process/coding-style.html

There is a script avail to check this called: scripts/checkpatch.pl
It summary says:
 total: 139 errors, 238 warnings, 6 checks, 372 lines checked
(Not good, more error+warnings than lines...)

Please fix up those...

>  include/net/ip_fib.h   |  40 +++++++
>  net/ipv4/Makefile      |   2 +-
>  net/ipv4/fib_poptrie.c | 295 +++++++++++++++++++++++++++++++++++++++++++++++++
>  net/ipv4/fib_trie.c    |   3 +
>  4 files changed, 339 insertions(+), 1 deletion(-)
>  create mode 100644 net/ipv4/fib_poptrie.c
[...]

> diff --git a/net/ipv4/fib_poptrie.c b/net/ipv4/fib_poptrie.c
> new file mode 100644
> index 0000000..b3a88ab
> --- /dev/null
> +++ b/net/ipv4/fib_poptrie.c
> @@ -0,0 +1,295 @@
[....]
> +/*Insert a new node at index*/
> +static void insert_chield_node(struct poptrie_node *node,
> +        char index)
> +{

You are misspelling "child" as "chield"

> +    int i, j;
> +    struct poptrie_node *arr;
> +    int arr_size  = (int)hweight64(node->nodevec);
> +
> +    arr = kcalloc(arr_size + 1, sizeof(*arr), GFP_ATOMIC);
> +    for (i = 0, j = 0; i < (arr_size + 1); i++) {
> +        if (i != index && j < arr_size)
> +            arr[i] = node->chield_nodes[j++];
> +    }
> +
> +    kfree(node->chield_nodes);
> +    node->chield_nodes = arr;
> +}

[...]
> +/*We assume that pt->root is not NULL*/
> +void poptrie_lookup(struct poptrie *pt, __be32 dest, struct net_device **dev)
> +{
> +    register u32 index;
> +    register u64 bitmap, bitmask;
> +    register unsigned long leaf_index;
> +    register unsigned long node_index;
> +    register struct poptrie_node *node = pt->root;
> +    register u8 fib_index = pt->def_nh;
> +    register u8 carry = 0;
> +    register u8 carry_bit = 2;

Do you have performance data, that tell you that "register" is needed here?

> +    while (1) {
> +        /*Extract 6 bytes from dest */
> +        if (likely(carry_bit != 8)) {
> +            index = ((dest & 252) >> carry_bit) | carry;
> +            carry = (dest & ((1 << carry_bit) - 1)) << (6 - carry_bit);
> +            carry_bit = carry_bit + 2;
> +            dest = dest >> 8;
> +        } else {
> +            index = carry;
> +            carry = 0;
> +            carry_bit = 2;
> +        }
> +
> +        /*Create a bitmap based on the the extracted value*/
> +        bitmap = 1ULL << index;
> +        bitmask = bitmap - 1;
> +
> +        /*Find corresponding leaf*/
> +        if (likely(node->vector & bitmap)) {
> +            leaf_index = hweight64(node->leafvec & bitmask);

Just as help for reviewers, the popcnt instruction is here.

 hweight64 == popcnt

> +            if (!(node->leafvec & bitmap))
> +                leaf_index--;
> +            fib_index = node->leaves[leaf_index];
> +        }
> +
> +        /*Find corresponding node*/
> +        if (likely(node->nodevec & bitmap)) {
> +            node_index = hweight64(node->nodevec & bitmask);

And here.

> +            node = &node->chield_nodes[node_index];
> +            continue;
> +        }
> +
> +        *dev = get_fib(&pt->nhs, fib_index);
> +        return;
> +    }
> +}

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer