Large number of VMAs for large ZGC heap

Tue Apr 25 15:31:34 UTC 2023

Hi Stefan,

thanks a lot for your answers. Wrt THPs, yes, it would be wise to use
explicit huge pages.

Does the single ZUnmapper thread compete with all mutator threads for the
page allocator?

Thanks, Thomas

On Tue, Apr 25, 2023 at 2:59 PM Stefan Karlsson <stefan.karlsson at oracle.com>
wrote:

> Hi Thomas,
>
> On 2023-04-25 09:58, Thomas Stüfe wrote:
>
> Hi ZGC experts,
>
> I see a strangeness with one of our customers running JDK 17 with ZGC, THP
> enabled (always), and a large heap of 4.6TB.
>
>
> Side-note: be careful about using THP and expecting good latencies, but if
> you do want to use THP with ZGC make sure to also change:
>
> /sys/kernel/mm/transparent_hugepage/shmem_enabled
>
> https://wiki.openjdk.org/display/zgc
>
>
> The number of VMAs exceeds 20 million. I try to understand whether that is
> normal or pathological.
>
> Looking at maps, I see millions of adjacent VMAs that point into the heap
> to different offsets:
>
> ```
> 15fc5f600000-15fc5f800000 rw-s 24630400000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 15fc5f800000-15fc5fa00000 rw-s 2504e600000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 15fc5fa00000-15fc5fc00000 rw-s 25330000000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 15fc5fc00000-15fc5fe00000 rw-s 26324200000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 15fc5fe00000-15fc60000000 rw-s 26f03a00000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> ```
>
> The different offsets prevent these mappings from being folded.
>
> The number of mappings surpasses what would be needed to map the heap.
> Almost all are 2MB mappings:
>
> Total number of mappings: 18634289
> Number of 2MB mappings:        18529201
> Per color: 6211420 / 6211429 / 6211439
>
> The total address space covered by these 2MB mappings is 38TB. Taking into
> account the triple-mapping, we still map about 12TB per color. That far
> exceeds the necessary room for a 4.6TB heap.
>
>
> ZGC reserves a larger address space for the heap than the given max heap
> size. This is done to make it easier to deal with large objects. There are
> some hints to the address space layout here:
>
> https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/cpu/x86/gc/z/zGlobals_x86.cpp
>
>
> Examining the mappings, I see that many offsets into the heap are mapped
> to multiple points, even discounting the triple mapping. For example,
> offset 105fe800000 is mapped six times per color, for a total of 12 times:
>
> 13438de00000-13438e000000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 15bf79400000-15bf79600000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 165022800000-165022a00000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 16fdad200000-16fdad400000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 17b1b9600000-17b1b9800000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 1d9860000000-1d9860200000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
>
> 23438de00000-23438e000000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 25bf79400000-25bf79600000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 265022800000-265022a00000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 26fdad200000-26fdad400000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 27b1b9600000-27b1b9800000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 2d9860000000-2d9860200000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
>
> 43438de00000-43438e000000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 45bf79400000-45bf79600000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 465022800000-465022a00000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 46fdad200000-46fdad400000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 47b1b9600000-47b1b9800000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
> 4d9860000000-4d9860200000 rw-s 105fe800000 00:0f 373323680
> /memfd:java_heap.hugetlb (deleted)
>
>
> What I think happens here is that when we detach virtual-to-physical
> memory mappings we don't do it immediately, instead the memory is handed
> over to a separate ZUnmapper thread. If that thread gets starved, typically
> because of an over provisioned machine, then these mappings start to build
> up. You can see the ZUnmapper code here:
>
> https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/share/gc/z/zUnmapper.cpp
>
> I recently looked into this and thought that the starvation happened
> because of how we take the lock for every ZPage we want to unmap. I
> prototyped a way to bulk fetch all pages, but that didn't seem to help.
> AFAICT, the big problem for us was still that the ZUnmapper thread was
> starved out. The prototype is here:
> https://github.com/stefank/jdk/tree/zgc_generational_bulk_unmapper
>
> You can can actually see this problem if you monitor the amount of
> committed memory in the Java heap. When this happens the reported amount of
> committed memory increases and can even go past the max heap size. This is
> a bug because of how report our virtual memory to NMT. I created a bug for
> that:
> https://bugs.openjdk.org/browse/JDK-8306841
>
> And a prototype:
>
> https://github.com/stefank/jdk/tree/zgc_generational_fix_nmt_overcommit_reporting
>
> The ZGC Page table contains close to a million ZGC pages and looks okay
> for a heap of that size:
> Small: 739175
> Medium: 10160
> Large:   65495
>                -------
>                 814830
>
> My question: is such a high number of mappings for ZGC normal?
>
>
> A larger number of mappings is normal, but what you have above indicates
> some kind of performance issue with the system.
>
> Cheers,
> StefanK
>
>
> Thank you for your time,
>
> Cheers, Thomas
>
>
>
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