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
