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Description of problem:
If qemu is configured with large amounts of hotpluggable memory (say > 256G) it takes a very long time to start the guest.
Version-Release number of selected component (if applicable):
qemu-kvm-rhev-2.3.0-22.el7.ppc64le
How reproducible:
100%
Steps to Reproduce:
1. Start qemu with
/usr/libexec/qemu-kvm -M pseries -m 512M,maxmem=XXXXX ...
For maxmem=512G, 1024G, 2048G
2. Time how long until qemu starts booting the guest
Actual results:
512G: ~17s
1024G: ~2m30s
2048G: ~20m
Expected results:
<10s startup in all cases.
Additional info:
The root cause is that qemu constructs a DR connector object internally for every 256M of hotpluggable memory. The current algorithm for doing so is O(n^3).
I've posted a patch upstream which improves adding the connectors to O(n^2). This brings startup with 2048G to an acceptable 4s, unfortunately 4096G (RHEV's usual limit) is still ~35s.
(In reply to David Gibson from comment #5)
> I've made draft brew build with the partial fix for slow startup at:
>
> https://brewweb.devel.redhat.com/taskinfo?taskID=9835517
Test with the above build, boot guest with the following configuration:
Maxmem:
256G: ~ 10s
512G: ~10s
1024G: ~15S
2048G: After 5s, hit the issue in bug 1263039 comment 4.
And this bug could be reproduced on the official build qemu-kvm-rhev-2.3.0-22.el7.ppc64le:
Maxmem:
256G: ~ 10s
512G: ~17s
1024G: 2~3 mins
2048G: after about 27 mins, hit bug 1263039.
Anyway, this qemu-kvm build fixes the original issue in this bug.
Comment 9Miroslav Rezanina
2015-09-18 11:54:29 UTC
Verified this bug with the following version:
kernel-3.10.0-316.el7.ppc64le
qemu-kvm-rhev-2.3.0-25.el7.ppc64le
SLOF-20150313-5.gitc89b0df.el7.noarch
Test with the same command line with comment 2 and the result is:
Maxmem:
256G: ~ 10s
512G: ~10s without guest desktop (with guest desktop, it consumes 20+ seconds)
1024G: ~10s without guest desktop (with guest desktop, it consumes 20+ seconds)
According to bug 1263039 comment6, we limit the maxmem to 1T.
So according to comment 8 and the above result, I will set the status to VERIFIED.
------- Comment From fnovak.com 2015-11-30 15:17 EDT-------
reverse mirror of RHBZ 1262143 - VM startup is very slow with large amounts of hotpluggable memory
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.
https://rhn.redhat.com/errata/RHBA-2015-2546.html
Description of problem: If qemu is configured with large amounts of hotpluggable memory (say > 256G) it takes a very long time to start the guest. Version-Release number of selected component (if applicable): qemu-kvm-rhev-2.3.0-22.el7.ppc64le How reproducible: 100% Steps to Reproduce: 1. Start qemu with /usr/libexec/qemu-kvm -M pseries -m 512M,maxmem=XXXXX ... For maxmem=512G, 1024G, 2048G 2. Time how long until qemu starts booting the guest Actual results: 512G: ~17s 1024G: ~2m30s 2048G: ~20m Expected results: <10s startup in all cases. Additional info: The root cause is that qemu constructs a DR connector object internally for every 256M of hotpluggable memory. The current algorithm for doing so is O(n^3).