Hi,
On Thu, Sep 6, 2018 at 8:32 PM, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Sep 06, 2018 at 12:33:58PM -0700, Eric Dumazet wrote:
>> On Thu, Sep 6, 2018 at 12:21 PM Olof Johansson <olof@xxxxxxxxx> wrote:
>> >
>> > Today these are all global shared variables per protocol, and in
>> > particular tcp_memory_allocated can get hot on a system with
>> > large number of CPUs and a substantial number of connections.
>> >
>> > Moving it over to a per-cpu variable makes it significantly cheaper,
>> > and the added overhead when summing up the percpu copies is still smaller
>> > than the cost of having a hot cacheline bouncing around.
>>
>> I am curious. We never noticed contention on this variable, at least for TCP.
>
> Yes these variables are heavily amortised so I'm surprised that
> they would cause much contention.
>
>> Please share some numbers with us.
>
> Indeed.
Certainly, just had to collect them again.
This is on a dual xeon box, with ~150-200k TCP connections. I see
about .7% CPU spent in __sk_mem_{reduce,raise}_allocated in the
inlined atomic ops, most of those in reduce.
Call path for reduce is practically all from tcp_write_timer on softirq:
__sk_mem_reduce_allocated
tcp_write_timer
call_timer_fn
run_timer_softirq
__do_softirq
irq_exit
smp_apic_timer_interrupt
apic_timer_interrupt
cpuidle_enter_state
With this patch, I see about .18+.11+.07 = .36% in percpu-related
functions called from the same __sk_mem functions.
Now, that's a halving of cycles samples on that specific setup. The
real difference though, is on another platform where atomics are more
expensive. There, this makes a significant difference. Unfortunately,
I can't share specifics but I think this change stands on its own on
the dual xeon setup as well, maybe with slightly less strong wording
on just how hot the variable/line happens to be.
-Olof
