<div dir="ltr"><div>Hi Stefan,</div><div><br></div><div>thanks a lot for your answers. Wrt THPs, yes, it would be wise to use explicit huge pages. <br></div><div><br></div><div>Does the single ZUnmapper thread compete with all mutator threads for the page allocator? <br></div><div><br></div><div>Thanks, Thomas<br></div><div><br></div><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Apr 25, 2023 at 2:59 PM Stefan Karlsson <<a href="mailto:stefan.karlsson@oracle.com">stefan.karlsson@oracle.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
<div>
<font face="monospace">Hi Thomas,</font><br>
<br>
<div>On 2023-04-25 09:58, Thomas Stüfe
wrote:<br>
</div>
<blockquote type="cite">
<div dir="ltr">Hi ZGC experts,<br>
<br>
I see a strangeness with one of our customers running JDK 17
with ZGC, THP enabled (always), and a large heap of 4.6TB. <br>
</div>
</blockquote>
<br>
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:<br>
<code><br>
/sys/kernel/mm/transparent_hugepage/shmem_enabled<br>
<br>
<a href="https://wiki.openjdk.org/display/zgc" target="_blank">https://wiki.openjdk.org/display/zgc</a><br>
</code><br>
<blockquote type="cite">
<div dir="ltr"><br>
The number of VMAs exceeds 20 million. I try to understand
whether that is normal or pathological.<br>
<br>
Looking at maps, I see millions of adjacent VMAs that point into
the heap to different offsets:<br>
<br>
```<br>
15fc5f600000-15fc5f800000 rw-s 24630400000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
15fc5f800000-15fc5fa00000 rw-s 2504e600000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
15fc5fa00000-15fc5fc00000 rw-s 25330000000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
15fc5fc00000-15fc5fe00000 rw-s 26324200000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
15fc5fe00000-15fc60000000 rw-s 26f03a00000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
```<br>
<br>
The different offsets prevent these mappings from being folded.<br>
<br>
The number of mappings surpasses what would be needed to map the
heap. Almost all are 2MB mappings:<br>
<br>
Total number of mappings: 18634289<br>
Number of 2MB mappings: 18529201<br>
Per color: 6211420 / 6211429 / 6211439<br>
<br>
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.<br>
</div>
</blockquote>
<br>
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:<br>
<a href="https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/cpu/x86/gc/z/zGlobals_x86.cpp" target="_blank">https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/cpu/x86/gc/z/zGlobals_x86.cpp</a><br>
<br>
<blockquote type="cite">
<div dir="ltr"><br>
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:<br>
<br>
13438de00000-13438e000000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
15bf79400000-15bf79600000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
165022800000-165022a00000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
16fdad200000-16fdad400000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
17b1b9600000-17b1b9800000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
1d9860000000-1d9860200000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
<br>
23438de00000-23438e000000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
25bf79400000-25bf79600000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
265022800000-265022a00000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
26fdad200000-26fdad400000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
27b1b9600000-27b1b9800000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
2d9860000000-2d9860200000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
<br>
43438de00000-43438e000000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
45bf79400000-45bf79600000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
465022800000-465022a00000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
46fdad200000-46fdad400000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
47b1b9600000-47b1b9800000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
4d9860000000-4d9860200000 rw-s 105fe800000 00:0f 373323680
/memfd:java_heap.hugetlb (deleted)<br>
<br>
</div>
</blockquote>
<br>
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:<br>
<a href="https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/share/gc/z/zUnmapper.cpp" target="_blank">https://github.com/openjdk/zgc/blob/5ea960728c5616373c986ae1343b44043c0db487/src/hotspot/share/gc/z/zUnmapper.cpp</a><br>
<br>
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:<br>
<a href="https://github.com/stefank/jdk/tree/zgc_generational_bulk_unmapper" target="_blank">https://github.com/stefank/jdk/tree/zgc_generational_bulk_unmapper</a><br>
<br>
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:<br>
<a href="https://bugs.openjdk.org/browse/JDK-8306841" target="_blank">https://bugs.openjdk.org/browse/JDK-8306841</a><br>
<br>
And a prototype:<br>
<a href="https://github.com/stefank/jdk/tree/zgc_generational_fix_nmt_overcommit_reporting" target="_blank">https://github.com/stefank/jdk/tree/zgc_generational_fix_nmt_overcommit_reporting</a><br>
<br>
<blockquote type="cite">
<div dir="ltr">The ZGC Page table contains close to a million ZGC
pages and looks okay for a heap of that size:<br>
Small: 739175<br>
Medium: 10160<br>
Large: 65495<br>
-------<br>
814830<br>
<br>
My question: is such a high number of mappings for ZGC normal?<br>
</div>
</blockquote>
<br>
A larger number of mappings is normal, but what you have above
indicates some kind of performance issue with the system.<br>
<br>
Cheers,<br>
StefanK<br>
<br>
<blockquote type="cite">
<div dir="ltr"><br>
Thank you for your time,<br>
<br>
Cheers, Thomas</div>
</blockquote>
<br>
</div>
</blockquote></div>