Re: [PATCH v4] ipc/msg: mitigate the lock contention with percpu counter

["Sun, Jiebin" <jiebin.sun@intel.com>]

On 9/8/2022 5:34 AM, Andrew Morton wrote:
> On Wed, 07 Sep 2022 09:01:53 -0700 Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
>> On Thu, 2022-09-08 at 01:25 +0800, Jiebin Sun wrote:
>>> The msg_bytes and msg_hdrs atomic counters are frequently
>>> updated when IPC msg queue is in heavy use, causing heavy
>>> cache bounce and overhead. Change them to percpu_counter
>>> greatly improve the performance. Since there is one percpu
>>> struct per namespace, additional memory cost is minimal.
>>> Reading of the count done in msgctl call, which is infrequent.
>>> So the need to sum up the counts in each CPU is infrequent.
>>>
>>>
>>> Apply the patch and test the pts/stress-ng-1.4.0
>>> -- system v message passing (160 threads).
>>>
>>> Score gain: 3.17x
>>>
>>>
>> ...
>>>   
>>> +/* large batch size could reduce the times to sum up percpu counter */
>>> +#define MSG_PERCPU_COUNTER_BATCH 1024
>>> +
>> Jiebin,
>>
>> 1024 is a small size (1/4 page).
>> The local per cpu counter could overflow to the gloabal count quickly
>> if it is limited to this size, since our count tracks msg size.
>>    
>> I'll suggest something larger, say 8*1024*1024, about
>> 8MB to accommodate about 2 large page worth of data.  Maybe that
>> will further improve throughput on stress-ng by reducing contention
>> on adding to the global count.
>>
> I think this concept of a percpu_counter_add() which is massively
> biased to the write side and with very rare reading is a legitimate
> use-case.  Perhaps it should become an addition to the formal interface.
> Something like
>
> /*
>   * comment goes here
>   */
> static inline void percpu_counter_add_local(struct percpu_counter *fbc,
> 					    s64 amount)
> {
> 	percpu_counter_add_batch(fbc, amount, INT_MAX);
> }
>
> and percpu_counter_sub_local(), I guess.
>
> The only instance I can see is
> block/blk-cgroup-rwstat.h:blkg_rwstat_add() which is using INT_MAX/2
> because it always uses percpu_counter_sum_positive() on the read side.
>
> But that makes two!


Yes. Using INT_MAX or INT_MAX/2 could have a big improvement on the 
performance if heavy writing but rare reading. In our case, if the local 
percpu counter is near to INT_MAX and there comes a big msgsz, the 
overflow issue could happen. So I think INT_MAX/2, which is used in 
blkg_rwstat_add(), might be a better choice. /$ 
percpu_counter_add_batch(&ns->percpu_msg_bytes, msgsz, batch); /I will 
send the performance data and draft patch out for discussing.//Jiebin//