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Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBack
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend |
Date: |
Tue, 10 Jun 2014 14:35:28 +0300 |
On Tue, Jun 10, 2014 at 01:29:06PM +0200, Paolo Bonzini wrote:
> Il 10/06/2014 13:23, Michael S. Tsirkin ha scritto:
> >On Tue, Jun 10, 2014 at 01:12:04PM +0200, Paolo Bonzini wrote:
> >>Il 10/06/2014 11:59, Michael S. Tsirkin ha scritto:
> >>>>>What's the point compared to memory-backend-ram?
> >>>>
> >>>>That you can use shared memory, for example together with vhost-user.
> >>>
> >>>I don't think it's a good idea until THP supports shared memory.
> >>
> >>Why? For example it would be useful for testing on machines that you don't
> >>have root for, and that do not have a hugetlbfs mount point. For example
> >>you could run the test case from the vhost-user's patches.
> >
> >Sounds useful, I guess we could allow this when running under qtest.
>
> Or TCG, or Xen. At this point, why single out KVM?
>
> (Also, "--enable-kvm -mem-path /dev/shm" works on 2.0, and it would be a
> regression in 2.1).
It prints
fprintf(stderr, "Warning: path not on HugeTLBFS: %s\n", path);
Correct?
I guess I agree then, hopefully the warning is enough.
Maybe add an extra warning that performance will suffer.
> >>THP is not a magic wand and you can get slowness from memory fragmentation
> >>at any time.
> >
> >Right but there's a difference between "can get slowness when memory
> >is overcommitted" and "will get slowness even on a mostly idle box".
>
> I would like to see the slowness on a real-world benchmark though. I
> suspect in most scenarios it would not matter.
Weird. Things like kernel build time are known to be measureably
improved by using THP.
> >>We should not limit ourselves due to kernel bugs.
> >
> >Why not? Practically people do have to run this on some kernel,
> >we should not use kernel in a way that it can't support well.
> >Old firefox doing a ton of fsync commands and slowing
> >the box to a crawl comes to mind as another example of this.
>
> Yes, and firefox doesn't say "no sorry can't do it" when running on such a
> kernel (which is much worse than more expensive TLB misses).
>
> Paolo
kernel can't speed up fsync. So IIRC instead, firefox switched to using
renames instead of fsync. IMHO QEMU should do the same, look for a
mechanism that kernel can support efficiently, instead of
insisting on using a feature that it can't.
--
MST
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, (continued)
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Igor Mammedov, 2014/06/09
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Hu Tao, 2014/06/09
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Hu Tao, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Hu Tao, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Michael S. Tsirkin, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Michael S. Tsirkin, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Michael S. Tsirkin, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Paolo Bonzini, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Igor Mammedov, 2014/06/10
- Re: [Qemu-devel] [PATCH v4 18/29] hostmem: add file-based HostMemoryBackend, Michael S. Tsirkin, 2014/06/10
[Qemu-devel] [PATCH v4 19/29] hostmem: add merge and dump properties, Hu Tao, 2014/06/09
[Qemu-devel] [PATCH v4 20/29] hostmem: allow preallocation of any memory region, Hu Tao, 2014/06/09
[Qemu-devel] [PATCH v4 21/29] hostmem: add property to map memory with MAP_SHARED, Hu Tao, 2014/06/09