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

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

On Wed, 2022-05-11 at 11:40 +0800, Aaron Lu wrote:
> On Tue, May 10, 2022 at 02:23:28PM +0800, ying.huang@intel.com wrote:
> > 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.
> 
> I've run the tests on a 2 sockets Icelake server and a Skylake desktop.
> 
> On this 2 sockets Icelake server(1.25MiB L2 = 320 pages, 48MiB LLC =
> 12288 pages):
> 
> pcp->high      score
>     0          100662 (bypass PCP, most page resue, most core-2-core transfer)
>   320(L2)      117252
>   640          133149
>  6144(1/2 llc) 134674
> 12416(>llc)    103193 (least page reuse)
> 
> Setting pcp->high to 640(2 times L2 size) gives very good result, only
> slightly lower than 6144(1/2 llc size). Bypassing PCP to get the most
> cache reuse didn't deliver good performance, so I think Ying is right:
> core-2-core really hurts.
> 
> On this 4core/8cpu Skylake desktop(256KiB L2 = 64 pages, 8MiB LLC = 2048
> pages):
> 
>    0           86780 (bypass PCP, most page reuse, most core-2-core transfer)
>   64(L2)       85813
>  128           85521
> 1024(1/2 llc)  85557
> 2176(> llc)    74458 (least page reuse)
> 
> Things are different on this small machine. Bypassing PCP gives the best
> performance. I find it hard to explain this. Maybe the 256KiB is too
> small that even bypassing PCP, the page still ends up being evicted from
> L2 when allocator side reuses it? Or maybe core-2-core transfer is
> fast on this small machine?

86780 / 85813 = 1.011

So, there's almost no measurable difference among the configurations
except the last one.  I would rather say the test isn't sensitive to L2
size, but sensitive to LLC size on this machine.

Best Regards,
Huang, Ying

> P.S. I've blindly setting pcp->high to the above value, ignoring zone's
> low watermark cap for testing purpose.