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Message-ID: <a58ad76f-780e-42de-86b3-44e24164d945@dorminy.me>
Date: Thu, 28 Mar 2024 12:46:06 -0400
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Subject: Re: [PATCH] fuse: increase FUSE_MAX_MAX_PAGES limit
To: Bernd Schubert <bernd.schubert@fastmail.fm>,
 Jingbo Xu <jefflexu@linux.alibaba.com>, Miklos Szeredi <miklos@szeredi.hu>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
 zhangjiachen.jaycee@bytedance.com
References: <20240124070512.52207-1-jefflexu@linux.alibaba.com>
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From: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
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On 3/7/24 17:06, Bernd Schubert wrote:
> Hi Jingbo,
> 
> On 3/7/24 03:16, Jingbo Xu wrote:
>> Hi Bernd,
>>
>> On 3/6/24 11:45 PM, Bernd Schubert wrote:
>>>
>>>
>>> On 3/6/24 14:32, Jingbo Xu wrote:
>>>>
>>>>
>>>> On 3/5/24 10:26 PM, Miklos Szeredi wrote:
>>>>> On Mon, 26 Feb 2024 at 05:00, Jingbo Xu <jefflexu@linux.alibaba.com> wrote:
>>>>>>
>>>>>> Hi Miklos,
>>>>>>
>>>>>> On 1/26/24 2:29 PM, Jingbo Xu wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 1/24/24 8:47 PM, Jingbo Xu wrote:
>>>>>>>>
>>>>>>>>
>>>>>>>> On 1/24/24 8:23 PM, Miklos Szeredi wrote:
>>>>>>>>> On Wed, 24 Jan 2024 at 08:05, Jingbo Xu <jefflexu@linux.alibaba.com> wrote:
>>>>>>>>>>
>>>>>>>>>> From: Xu Ji <laoji.jx@alibaba-inc.com>
>>>>>>>>>>
>>>>>>>>>> Increase FUSE_MAX_MAX_PAGES limit, so that the maximum data size of a
>>>>>>>>>> single request is increased.
>>>>>>>>>
>>>>>>>>> The only worry is about where this memory is getting accounted to.
>>>>>>>>> This needs to be thought through, since the we are increasing the
>>>>>>>>> possible memory that an unprivileged user is allowed to pin.
>>>>>>>
>>>>>>> Apart from the request size, the maximum number of background requests,
>>>>>>> i.e. max_background (12 by default, and configurable by the fuse
>>>>>>> daemon), also limits the size of the memory that an unprivileged user
>>>>>>> can pin.  But yes, it indeed increases the number proportionally by
>>>>>>> increasing the maximum request size.
>>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> This optimizes the write performance especially when the optimal IO size
>>>>>>>>>> of the backend store at the fuse daemon side is greater than the original
>>>>>>>>>> maximum request size (i.e. 1MB with 256 FUSE_MAX_MAX_PAGES and
>>>>>>>>>> 4096 PAGE_SIZE).
>>>>>>>>>>
>>>>>>>>>> Be noted that this only increases the upper limit of the maximum request
>>>>>>>>>> size, while the real maximum request size relies on the FUSE_INIT
>>>>>>>>>> negotiation with the fuse daemon.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Xu Ji <laoji.jx@alibaba-inc.com>
>>>>>>>>>> Signed-off-by: Jingbo Xu <jefflexu@linux.alibaba.com>
>>>>>>>>>> ---
>>>>>>>>>> I'm not sure if 1024 is adequate for FUSE_MAX_MAX_PAGES, as the
>>>>>>>>>> Bytedance floks seems to had increased the maximum request size to 8M
>>>>>>>>>> and saw a ~20% performance boost.
>>>>>>>>>
>>>>>>>>> The 20% is against the 256 pages, I guess.
>>>>>>>>
>>>>>>>> Yeah I guess so.
>>>>>>>>
>>>>>>>>
>>>>>>>>> It would be interesting to
>>>>>>>>> see the how the number of pages per request affects performance and
>>>>>>>>> why.
>>>>>>>>
>>>>>>>> To be honest, I'm not sure the root cause of the performance boost in
>>>>>>>> bytedance's case.
>>>>>>>>
>>>>>>>> While in our internal use scenario, the optimal IO size of the backend
>>>>>>>> store at the fuse server side is, e.g. 4MB, and thus if the maximum
>>>>>>>> throughput can not be achieved with current 256 pages per request. IOW
>>>>>>>> the backend store, e.g. a distributed parallel filesystem, get optimal
>>>>>>>> performance when the data is aligned at 4MB boundary.  I can ask my folk
>>>>>>>> who implements the fuse server to give more background info and the
>>>>>>>> exact performance statistics.
>>>>>>>
>>>>>>> Here are more details about our internal use case:
>>>>>>>
>>>>>>> We have a fuse server used in our internal cloud scenarios, while the
>>>>>>> backend store is actually a distributed filesystem.  That is, the fuse
>>>>>>> server actually plays as the client of the remote distributed
>>>>>>> filesystem.  The fuse server forwards the fuse requests to the remote
>>>>>>> backing store through network, while the remote distributed filesystem
>>>>>>> handles the IO requests, e.g. process the data from/to the persistent store.
>>>>>>>
>>>>>>> Then it comes the details of the remote distributed filesystem when it
>>>>>>> process the requested data with the persistent store.
>>>>>>>
>>>>>>> [1] The remote distributed filesystem uses, e.g. a 8+3 mode, EC
>>>>>>> (ErasureCode), where each fixed sized user data is split and stored as 8
>>>>>>> data blocks plus 3 extra parity blocks. For example, with 512 bytes
>>>>>>> block size, for each 4MB user data, it's split and stored as 8 (512
>>>>>>> bytes) data blocks with 3 (512 bytes) parity blocks.
>>>>>>>
>>>>>>> It also utilize the stripe technology to boost the performance, for
>>>>>>> example, there are 8 data disks and 3 parity disks in the above 8+3 mode
>>>>>>> example, in which each stripe consists of 8 data blocks and 3 parity
>>>>>>> blocks.
>>>>>>>
>>>>>>> [2] To avoid data corruption on power off, the remote distributed
>>>>>>> filesystem commit a O_SYNC write right away once a write (fuse) request
>>>>>>> received.  Since the EC described above, when the write fuse request is
>>>>>>> not aligned on 4MB (the stripe size) boundary, say it's 1MB in size, the
>>>>>>> other 3MB is read from the persistent store first, then compute the
>>>>>>> extra 3 parity blocks with the complete 4MB stripe, and finally write
>>>>>>> the 8 data blocks and 3 parity blocks down.
>>>>>>>
>>>>>>>
>>>>>>> Thus the write amplification is un-neglectable and is the performance
>>>>>>> bottleneck when the fuse request size is less than the stripe size.
>>>>>>>
>>>>>>> Here are some simple performance statistics with varying request size.
>>>>>>> With 4MB stripe size, there's ~3x bandwidth improvement when the maximum
>>>>>>> request size is increased from 256KB to 3.9MB, and another ~20%
>>>>>>> improvement when the request size is increased to 4MB from 3.9MB.
>>>>>
>>>>> I sort of understand the issue, although my guess is that this could
>>>>> be worked around in the client by coalescing writes.  This could be
>>>>> done by adding a small delay before sending a write request off to the
>>>>> network.
>>>>>
>>>>> Would that work in your case?
>>>>
>>>> It's possible but I'm not sure. I've asked my colleagues who working on
>>>> the fuse server and the backend store, though have not been replied yet.
>>>>   But I guess it's not as simple as increasing the maximum FUSE request
>>>> size directly and thus more complexity gets involved.
>>>>
>>>> I can also understand the concern that this may increase the risk of
>>>> pinning more memory footprint, and a more generic using scenario needs
>>>> to be considered.  I can make it a private patch for our internal product.
>>>>
>>>> Thanks for the suggestions and discussion.
>>>
>>> It also gets kind of solved in my fuse-over-io-uring branch - as long as
>>> there are enough free ring entries. I'm going to add in a flag there
>>> that other CQEs might be follow up requests. Really time to post a new
>>> version.
>>
>> Thanks for the information.  I've not read the fuse-over-io-uring branch
>> yet, but sounds like it would be much helpful .  Would there be a flag
>> in the FUSE request indicating it's one of the linked FUSE requests?  Is
>> this feature, say linked FUSE requests, enabled only when io-uring is
>> upon FUSE?
> 
> 
> Current development branch is this
> https://github.com/bsbernd/linux/tree/fuse-uring-for-6.8
> (It sometimes gets rebase/force pushes and incompatible changes - the
> corresponding libfuse branch is also persistently updated).
> 
> Patches need clean up before I can send the next RFC version. And I
> first want to change fixed single request size (not so nice to use 1MB
> requests when 4K would be sufficient, for things like metadata and small
> IO).
> 

Let me know if there's something you'd like collaboration on -- 
fuse_iouring sounds very exciting and I'd love to help out any way that 
would be useful.

For our internal usecase at Meta, the relevant backend store operates on 
8M chunks, so I'm also very interested in the simplicity of just opting 
in to receiving 8M IOs from the kernel instead of needing to buffer our 
own 8MB IOs. But io_uring does seem like a plausible general-purpose 
improvement too, so either or both of these paths is interesting and I'm 
working on gathering performance numbers on the relative merits.

Thanks!

Sweet Tea