Re: [mm/page_alloc] f26b3fa046: netperf.Throughput_Mbps -18.0% regression

["ying.huang@intel.com" <ying.huang@intel.com>]

On Tue, 2022-05-10 at 11:43 +0800, Aaron Lu wrote:
> On 5/7/2022 3:44 PM, ying.huang@intel.com wrote:
> > On Sat, 2022-05-07 at 15:31 +0800, Aaron Lu wrote:
> 
> ... ...
> 
> > > 
> > > I thought the overhead of changing the cache line from "shared" to
> > > "own"/"modify" is pretty cheap.
> > 
> > This is the read/write pattern of cache ping-pong.  Although it should
> > be cheaper than the write/write pattern of cache ping-pong in theory, we
> > have gotten sevious regression for that before.
> > 
> 
> Can you point me to the regression report? I would like to take a look,
> thanks.

Sure.

https://lore.kernel.org/all/1425108604.10337.84.camel@linux.intel.com/

> > > Also, this is the same case as the Skylake desktop machine, why it is a
> > > gain there but a loss here? 
> > 
> > I guess the reason is the private cache size.  The size of the private
> > L2 cache of SKL server is much larger than that of SKL client (1MB vs.
> > 256KB).  So there's much more core-2-core traffic on SKL server.
> > 
> 
> It could be. The 256KiB L2 in Skylake desktop can only store 8 order-3
> pages and that means the allocator side may have a higher chance of
> reusing a page that is evicted from the free cpu's L2 cache than the
> server machine, whose L2 can store 40 order-3 pages.
> 
> I can do more tests using different high for the two machines:
> 1) high=0, this is the case when page reuse is the extreme. core-2-core
> transfer should be the most. This is the behavior of this bisected commit.
> 2) high=L2_size, this is the case when page reuse is fewer compared to
> the above case, core-2-core should still be the majority.
> 3) high=2 times of L2_size and smaller than llc size, this is the case
> when cache reuse is further reduced, and when the page is indeed reused,
> it shouldn't cause core-2-core transfer but can benefit from llc.
> 4) high>llc_size, this is the case when page reuse is the least and when
> page is indeed reused, it is likely not in the entire cache hierarchy.
> This is the behavior of this bisected commit's parent commit for the
> Skylake desktop machine.
> 
> I expect case 3) should give us the best performance and 1) or 4) is the
> worst for this testcase.
> 
> case 4) is difficult to test on the server machine due to the cap of
> pcp->high which is affected by the low watermark of the zone. The server
> machine has 128 cpus but only 128G memory, which makes the pcp->high
> capped at 421, while llc size is 40MiB and that translates to a page
> number of 12288.
> > 

Sounds good to me.

Best Regards,
Huang, Ying

> > > Is it that this "overhead" is much greater
> > > in server machine to the extent that it is even better to use a totally
> > > cold page than a hot one?
> > 
> > Yes.  And I think the private cache size matters here.  And after being
> > evicted from the private cache (L1/L2), the cache lines of the reused
> > pages will go to shared cache (L3), that will help performance.
> > 
> 
> Sounds reasonable.
> 
> > > If so, it seems to suggest we should avoid
> > > cache reuse in server machine unless the two CPUs happens to be two
> > > hyperthreads of the same core.
> > 
> > Yes.  I think so.