ZGC heap size and RSS counters

[Per Liden]

Hi,

On 2017-12-11 09:55, Aleksey Shipilev wrote:
> Hi there,
>
> I'm trying ZGC on trivial workloads, and I have a question about footprint. The workload runs with
> -Xms16g -Xms16g, but the RSS figures are at least 3x larger:
>
> VmPeak:	18256721392 kB
> VmSize:	18256721392 kB
> VmLck:	       0 kB
> VmPin:	       0 kB
> VmHWM:	50729036 kB
> VmRSS:	50729036 kB
> RssAnon:	  369700 kB
> RssFile:	   27688 kB
> RssShmem:	50331648 kB
>
> Is this because ZGC maps the same physical space with multiple virtual mappings? Or is it a bug?

The kernel's RSS accounting is flaky at best, and varies depending on if 
you're using small or large pages (and it can also vary depending on 
which kernel version you're using).

On Linux/x86_64, we map the heap in three different locations. When 
using small pages, you'll typically see that the same physical page will 
incorrectly be accounted for three times instead of once. On the other 
hand, when using large pages, you'll typically see a different behavior, 
as it's accounted to the hugetlbfs inode and not the process.

In summary, it's not a bug in ZGC, but more a limitation in Linux's 
accounting.

/Per

>
> Thanks,
> -Aleksey
>