[ovs-dev] [PATCH 2/3] ofproto-dpif: Check frags only once.

[Jarno Rajahalme]

On Sep 26, 2014, at 11:33 AM, Ben Pfaff <blp at nicira.com> wrote:

> On Wed, Sep 24, 2014 at 11:24:01AM -0700, Jarno Rajahalme wrote:
>> Earlier I have seen the frags checking suspiciously high in perf
>> reports, but did not understand why.  Maybe this explains it:
>> 
>> Previously we checked frags handling before each flow table lookup.
>> As the nw_frags field is not writeable, it suffices to check them
>> once, before the first table lookup.  Also, ofproto-dpif-xlate already
>> has code for this, but it was run after the first table lookup.  This
>> check is now done only once, before the first table lookup.
>> 
>> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
> 
> By "frags checking suspiciously high in perf reports", do you mean
> these deleted lines of code in rule_dpif_lookup_in_table() were
> showing up high in perf reports?
> 
> -    if (ofproto->up.frag_handling != OFPC_FRAG_NX_MATCH) {
> -        /* We always unwildcard dl_type and nw_frag (for IP), so they
> -         * need not be unwildcarded here. */
> -
> -        if (flow->nw_frag & FLOW_NW_FRAG_ANY) {
> 

Yes they did. I could make it a bit better just by changing the order of the checks, checking flow->nw_frag first, and ofproto->ip.frag_handling later, so it must have been a cache miss on ofproto->up.frag_handling.

This was already some months ago, and I have not repeated the tests recently.

> I'm really surprised to hear that.
> 

To be clear, it was in the range of < 1%, so it was not that high, but I was surprised to see it there at all. Maybe I should just remove the word “high” from the commit message?

  Jarno

> But maybe you mean something else...?