[ovs-dev] [PATCH 1/3] [bug 2462] flex-array: Flex arrays-cleanup.

[Jesse Gross]

On Fri, Feb 3, 2012 at 5:45 AM, Pravin Shelar <pshelar at nicira.com> wrote:
> On Thu, Feb 2, 2012 at 7:05 PM, Jesse Gross <jesse at nicira.com> wrote:
>> It's not clear to me that flex_array_init() is that big of a win.
>> struct flex_array stores the base array inside of it, so it is defined
>> to be PAGE_SIZE.  Since it is so large, storing that inside of another
>> struct as you do in the third patch isn't necessarily a great idea.
>> We already allocate the port array separately, so it's not clear why
>> this is different.
>
> As vport flex array does not fit in a page, It needs to access
> *fa->parts[] element on lookup.
> By keeping struct flex_array inside struct datapath, I am trying to
> reduce extra address access.
> The lookup is done on every packet out and first few parts[] shld be
> in same cache-line so I think it helps.

It's large enough that it will push stats_percpu out of the same cache
line as the rest of struct datapath so either you will take a cache
miss accessing that or (hopefully) it will also be resident in the
cache because it is accessed frequently.  At best it's a wash, at
worst you miss on both this and the stats.  Without data, I think
these types of optimizations are premature.  If it does help (and I
know there are plenty of ways that we can improve our cache usage)
then we should do that independently, rather than as part of a
feature.