Re: [Question] performance regression after VM migration due to anon THP split in CoW

[David Hildenbrand <david@redhat.com>]

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Hi,

Likely the mailing lists won‘t like my mail from this Google Mail client ;)

Jinjiang Tu <tujinjiang@huawei.com> schrieb am Sa. 29. Juni 2024 um 11:18:

> Hi,
>
> We noticed a performance regression in benchmark memtester[1] after
> upgrading the kernel. THP is enabled by default
> (/sys/kernel/mm/transparent_hugepage/enabled
> is set to "always"). The issue arises when we migrate a virtual machine
> that has 125G total memory and 124G free memory to another host. And then,
> we run the command `memtester 120G` in the VM. The benchmark takes about
> 20 seconds to consume 120G memory in v4.18, but takes about 160 seconds in
> v5.10. This issue exists in mainline kernel too.
>

Simple: use preallocation in QEMU. „prealloc=on“ for host memory backends,
for example.


> We find commit 3917c80280c9 ("thp: change CoW semantics for anon-THP")
> leads to the performance regression. Since this commit, When we trigger a
> write fault on a anon THP, we split the PMD and allocate a 4K page, instead
> of allocating the full anon THP. When a VM is migrating (based on qemu[2]),
> if the page is marked zero page in the source VM, the destination VM will
> call mmap and read the region to allocate memory, making the region mapped
> by the zero THP. When we run memtester in the destination VM after VM
> migration finishes, memtester(in VM) will allocate large amounts of free
> memory and write to them, cause CoW of anon THP and THP split, further
> cause performance regression. After reverting this commit, performance
> regression disappears.


You talk about COW of anon THP, whereby your scenario really only relied on
COW of the huge zeropage.

Wouldn’t you would get a similar result when disabling the huge zeropage?


>
> This commit optimises some scenarios such as Redis, but may lead to
> performance regression in some other scenarios, such as VM migration.
> How could we solve this issue? Maybe we could add a new sysctl to let users
> decide whether to CoW the full anon THP or not?
>

I‘m not convinced the use case you present really warrants a toggle for
that. In your case you only want to change semantics on COW fault to the
huge zeropage. But …

Using preallocation in QEMU will give you all anon THP right from the
start, avoiding any cow. Sure, you consume all memory right away, but after
all that‘s what your use case triggers either way. And it might all be even
faster. :)

Cheers!


> Thanks.
>
> [1] https://github.com/jnavila/memtester/tree/master
> [2] https://github.com/qemu/qemu/blob/master/migration/ram.c
>
>
>

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