Re: [RFC 1/1] io_uring: improve register file feature's usability

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,


> On 10/12/21 14:11, Xiaoguang Wang wrote:
>>> On 10/12/21 09:48, Xiaoguang Wang wrote:
>>>> The idea behind register file feature is good and straightforward, but
>>>> there is a very big issue that it's hard to use for user apps. User 
>>>> apps
>>>> need to bind slot info to file descriptor. For example, user app wants
>>>> to register a file, then it first needs to find a free slot in 
>>>> register
>>>> file infrastructure, that means user app needs to maintain slot 
>>>> info in
>>>> userspace, which is a obvious burden for userspace developers.
>>>
>>> Slot allocation is specifically entirely given away to the userspace,
>>> the userspace has more info and can use it more efficiently, e.g.
>>> if there is only a small managed set of registered files they can
>>> always have O(1) slot "lookup", and a couple of more use cases.
>>
>> Can you explain more what is slot "lookup", thanks. For me, it seems 
>> that
>
> I referred to nothing particular, just a way userspace finds a new index,
> can be round robin or "index==fd".
>
>> use fd as slot is the simplest and most efficient way, user does not 
>> need to> mange slot info at all in userspace.
>
> As mentioned, it should be slightly more efficient to have a small table,
> cache misses. Also, it's allocated with kvcalloc() so if it can't be
> allocate physically contig memory it will set up virtual memory.
>
> So, if the userspace has some other way of indexing files, small tables
> are preferred. For instance if it operates with 1-2 files, or stores 
> files
> in an array and the index in the array may serve the purpose, or any 
> other
> way. Also, additional memory for those who care.

Yeah, I agree with you that for small tables, current implementation 
seems good,

If user app just registers a small number of files, it may handle it 
well, but imagine

how netty, nginx or other network apps which will open thousands of 
socket files,

manage these socket files' slot info will be a obvious burden to 
developer, these

apps may need to develop a private component to record used or free 
slot. Especially

in a high concurrency scenario, frequent sockes opened or closed, this 
private component

may need locks to protect, that means this private component will 
introduce overhead too.

For a fd, vfs layer has already ensure its unique.

>
>>> If userspace wants to mimic a fdtable into io_uring's registered table,
>>> it's possible to do as is and without extra fdtable tracking
>>>
>>> fd = open();
>>> io_uring_update_slot(off=fd, fd=fd);
>>
>> No, currently it's hard to do above work, unless we register a big 
>> number of files initially.
>
> If they intend to use a big number of files that's the way to go. They
> can unregister/register if needed, usual grow factor=2  should make
> it workable.

I'm not sure un-register/register are appropriate，say a app registers 
1000 files, then

it needs to un-register 1000 files firstly, there are doubts whether can 
do this un-registration

work, if some of these files are used by other threads, which submit 
sqes with FIXED_FILE

flags continually, so the first un-registration work needs to 
synchronize with threads which

are submitting requests. And later app needs to prepare a new files 
array, saving current 1000

files and new files info to this new array, for me, it can works, but 
not efficient and somewhat

hard to use :)

What I express here is that there are many factors to consider carefully 
when using file

registration feature, that's why I say it's somewhat hard to use :)


Do you know any popular apps based on io_uring that have used file 
registration feature ?

netty (https://github.com/netty/netty-incubator-transport-io_uring.git) 
has io_uring support,

but does not use file registration feature, and recently  we'd like to 
add file registration

to it.


Regards,

Xiaoguang Wang

>
> We may consider fast growing as a separate feature if really needed,
> either as you did it, or even better doing it explicitly and separately
> from updates.
>
>
>> Say we call IORING_REGISTER_FILES to register 1000 files initially, 
>> then the io_uring
>>
>> io_file_table only supports 1000 files, fd which is greater than 1000 
>> will be not able to
>>
>> be registered.
>>
>> For safety,  you may need to register the number of 
>> getrlimit(RLIMIT_NOFILE) initially,
>>
>> but it also may fail, user may change RLIMIT_NOFILE too. This is why 
>> I introduce a
>>
>> io_uring io_file_table resize feature, but I agree this method may 
>> waste memory, for
>>
>> example, user app only wants one file registered, but this file's fd 
>> is very large.
>
> That's fine as long as it's optional
>