ZGC heap size and RSS counters

Per Liden per.liden at oracle.com
Mon Dec 11 09:59:51 UTC 2017

On 2017-12-11 10:36, Aleksey Shipilev wrote:
> On 12/11/2017 10:14 AM, Per Liden wrote:
>> 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.
> Understood, that's what I thought. Do you think that is the problem in lieu of pervasive use of
> containers that allocate/limit resources based on RSS?

If RSS limits are used in a container, then I'd argue that the kernel 
better get the accounting right, otherwise those limits is fairly 
useless wouldn't you say? In the kernel's defense, it is gradually 
getting better in this area.


> Thanks,
> -Aleksey

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