hi,
On 11/4/20 8:20 PM, Xiaoguang Wang wrote:
hi,
In io_file_get() and io_put_file(), currently we use percpu_ref_get() and
percpu_ref_put() for registered files, but it's hard to say they're very
light-weight synchronization primitives, especially in arm platform. In one
our arm machine, I get below perf data(registered files enabled):
Samples: 98K of event 'cycles:ppp', Event count (approx.): 63789396810
Overhead Command Shared Object Symbol
...
0.78% io_uring-sq [kernel.vmlinux] [k] io_file_get
There is an obvious overhead that can not be ignored.
Currently I don't find any good and generic solution for this issue, but
in IOPOLL mode, given that we can always ensure get/put registered files
under uring_lock, we can use a simple and plain u64 counter to synchronize
with registered files update operations in __io_sqe_files_update().
With this patch, perf data show shows:
Samples: 104K of event 'cycles:ppp', Event count (approx.): 67478249890
Overhead Command Shared Object Symbol
...
0.27% io_uring-sq [kernel.vmlinux] [k] io_file_get
The above %0.78 => %0.27 improvements are observed in arm machine with
4.19 kernel. In upstream mainline codes, since this patch
"2b0d3d3e4fcf percpu_ref: reduce memory footprint of percpu_ref in
fast path", I believe the io_file_get's overhead would be further
smaller. I have same tests in same machine, in upstream codes with my
patch, now the io_file_get's overhead is %0.44.
This patch's idea is simple, and now seems it only gives minor
performance improvement, do you have any comments about this patch,
should I continue re-send it?
Can you resend it against for-5.11/io_uring? Looks simple enough to me,
and it's a nice little win.Thanks, I'll prepare V2 soon.
Regards,
Xiaoguang Wang
