Re: [Virtio-fs] [PATCH v4 0/8] fuse,virtiofs: support per-file DAX

Hi Vivek, Miklos,

On 9/16/21 4:21 PM, JeffleXu wrote:

Hi, I add some performance statistics below.


On 8/17/21 8:40 PM, Vivek Goyal wrote:

quoted

On Tue, Aug 17, 2021 at 10:32:14AM +0100, Dr. David Alan Gilbert wrote:

quoted

* Miklos Szeredi (miklos@szeredi.hu) wrote:

quoted

On Tue, 17 Aug 2021 at 04:22, Jeffle Xu [off-list ref] wrote:

quoted

This patchset adds support of per-file DAX for virtiofs, which is
inspired by Ira Weiny's work on ext4[1] and xfs[2].

Can you please explain the background of this change in detail?

Why would an admin want to enable DAX for a particular virtiofs file
and not for others?

Where we're contending on virtiofs dax cache size it makes a lot of
sense; it's quite expensive for us to map something into the cache
(especially if we push something else out), so selectively DAXing files
that are expected to be hot could help reduce cache churn.

Yes, the performance of dax can be limited when the DAX window is
limited, where dax window may be contended by multiple files.

I tested kernel compiling in virtiofs, emulating the scenario where a
lot of files contending dax window and triggering dax window reclaiming.

Environment setup:
- guest vCPU: 16
- time make vmlinux -j128

type    | cache  | cache-size | time
------- | ------ | ---------- | ----
non-dax | always |   --       | real 2m48.119s
dax     | always | 64M        | real 4m49.563s
dax     | always |   1G       | real 3m14.200s
dax     | always |   4G       | real 2m41.141s


It can be seen that there's performance drop, comparing to the normal
buffered IO, when dax window resource is restricted and dax window
relcaiming is triggered. The smaller the cache size is, the worse the
performance is. The performance drop can be alleviated and eliminated as
cache size increases.

Though we may not compile kernel in virtiofs, indeed we may access a lot
of small files in virtiofs and suffer this performance drop.

quoted

In that case probaly we should just make DAX window larger. I assume

Yes, as the DAX window gets larger, it is less likely that we can run
short of dax window resource.

However it doesn't come without cost. 'struct page' descriptor for dax
window will consume guest memory at a ratio of ~1.5% (64/4096 = ~1.5%,
page descriptor is of 64 bytes size, assuming 4K sized page). That is,
every 1GB cache size will cost 16MB guest memory. As the cache size
increases, the memory footprint for page descriptors also increases,
which may offset the benefit of dax by eliminating guest page cache.

In summary, per-file dax feature tries to achieve a balance between
performance and memory overhead, by offering a finer gained control for
dax to users.

I'm not sure if this is adequate for introducing per-file dax feature to
community? Need some feedback from the community.

And if that's the case, I also want to know if setting/clearing S_DAX
inside guest is needed, since in our internal using scenario, setting
S_DAX from host daemon is adequate. If setting/clearing S_DAX inside
guest can be omitted then, the negotiation during FUSE_INIT phase is not
needed either. After all we could completely rely on the FUSE_ATTR_DAX
flag feeded by host daemon to see if dax shall be enabled or not for
corresponding file. The whole patch set will also be somehow simper then.


-- 
Thanks,
Jeffle
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