Thanks for review.
On 4/1/2022 9:13 PM, Peter Xu wrote:
Chongyun,
On Mon, Mar 28, 2022 at 09:32:10AM +0800, wuc...@chinatelecom.cn wrote:
From: Chongyun Wu <wuc...@chinatelecom.cn>
v2:
-patch 1: remove patch_1
v1:
-rebase to qemu/master
Overview
============
This series of patches is to optimize the performance of
online migration using dirty ring and autoconverge.
Mainly through the following aspects to do optimization:
1. Dynamically adjust the dirty ring collection thread to
reduce the occurrence of ring full, thereby reducing the
impact on customers, improving the efficiency of dirty
page collection, and thus improving the migration efficiency.
2. When collecting dirty pages from KVM,
kvm_cpu_synchronize_kick_all is not called if the rate is
limited, and it is called only once before suspending the
virtual machine. Because kvm_cpu_synchronize_kick_all will
become very time-consuming when the CPU is limited, and
there will not be too many dirty pages, so it only needs
to be called once before suspending the virtual machine to
ensure that dirty pages will not be lost and the efficiency
of migration is guaranteed .
3. Based on the characteristic of collecting dirty pages
in the dirty ring, a new dirty page rate calculation method
is proposed to obtain a more accurate dirty page rate.
4. Use a more accurate dirty page rate and calculate the
matched speed limit throttle required to complete the
migration according to the current system bandwidth and
parameters, instead of the current time-consuming method
of trying to get a speed limit, greatly reducing migration
time.
Thanks for the patches.
I'm curious what's the relationship between this series and Yong's?
I personally think it is a complementary relationship. Yong's can limit
per-vcpu. In the case of memory pressure threads in certain vcpu scenarios, the
restrictions on other vcpus are very small, and the impact on customers during
the migration process will be smaller. The auto-convergence optimization of the
last two patches in this patch series can cope with scenarios where the memory
pressure is balanced on each vcpu. Each has its own advantages, and customers
can choose the appropriate mode according to their own application scenarios.
The first two patches are for the dirty ring, and both auto converge and yong
modes can improve performance.
If talking about throttling, I do think the old auto-converge was kind of
inefficient comparing to the new per-vcpu ways of throttling at least in
either granularity or on read tolerances (e.g., dirty ring based solution
will not block vcpu readers even if the thread is heavily throttled).
Yes, I agree with that. Through the research of dirty ring and a lot of tests,
some points that may affect the advantages of dirty ring have been found, so
some optimizations have been made, and these optimizations are found to be
effective through testing and verification.
In this patch series, only the last two patches are optimized for autocoverge.
The first two patches are for all situations where the dirty ring is used,
including Yong's, and there is no conflict with his. Among them, "kvm:
Dynamically adjust the rate of dirty ring reaper thread" is proposed to take
advantage of dirty ring. When the memory pressure is high, speeding up the rate
at which the reaper thread collects dirty pages can effectively solve the
problem that the frequent occurrence of ring full leads to the frequent exit of
the guest and the performance of the guestperf is degraded. When the migration
thread migrates data, it also completes the synchronization of most dirty pages.
When the migration thread of the dirty ring synchronizes the dirty pages, it
will take less time, which will also speed up the migration. These two patches
will make yong's test results better, and the two optimization points are different.
We've got quite a few techniques taking care of migration convergence
issues (didn't mention postcopy yet..), and I'm wondering whether at some
point we should be more focused and make a chosen one better, rather than
building different blocks servicing the same purpose.
I'm sorry, maybe I should separate these patch series to avoid
misunderstandings. These patches and yong's should be complementary, and two of
them can also help yong get some performance improvements.
Thanks,
--
Best Regard,
Chongyun Wu
