Re: Large number of empty reads on 5.9-rc2 under moderate load

[Jens Axboe <axboe@kernel.dk>]

On 8/24/20 10:13 AM, Dmitry Shulyak wrote:
> On Mon, 24 Aug 2020 at 19:10, Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 8/24/20 9:33 AM, Dmitry Shulyak wrote:
>>> On Mon, 24 Aug 2020 at 17:45, Jens Axboe <axboe@kernel.dk> wrote:
>>>>
>>>> On 8/24/20 8:06 AM, Jens Axboe wrote:
>>>>> On 8/24/20 5:09 AM, Dmitry Shulyak wrote:
>>>>>> library that i am using https://github.com/dshulyak/uring
>>>>>> It requires golang 1.14, if installed, benchmark can be run with:
>>>>>> go test ./fs -run=xx -bench=BenchmarkReadAt/uring_8 -benchtime=1000000x
>>>>>> go test ./fs -run=xx -bench=BenchmarkReadAt/uring_5 -benchtime=8000000x
>>>>>>
>>>>>> note that it will setup uring instance per cpu, with shared worker pool.
>>>>>> it will take me too much time to implement repro in c, but in general
>>>>>> i am simply submitting multiple concurrent
>>>>>> read requests and watching read rate.
>>>>>
>>>>> I'm fine with trying your Go version, but I can into a bit of trouble:
>>>>>
>>>>> axboe@amd ~/g/go-uring (master)>
>>>>> go test ./fs -run=xx -bench=BenchmarkReadAt/uring_8 -benchtime=1000000x
>>>>> # github.com/dshulyak/uring/fixed
>>>>> fixed/allocator.go:38:48: error: incompatible type for field 2 in struct construction (cannot use type uint64 as type syscall.Iovec_len_t)
>>>>>    38 |  iovec := []syscall.Iovec{{Base: &mem[0], Len: uint64(size)}}
>>>>>       |                                                ^
>>>>> FAIL  github.com/dshulyak/uring/fs [build failed]
>>>>> FAIL
>>>>> axboe@amd ~/g/go-uring (master)> go version
>>>>> go version go1.14.6 gccgo (Ubuntu 10.2.0-5ubuntu1~20.04) 10.2.0 linux/amd64
>>>>
>>>> Alright, got it working. What device are you running this on? And am I
>>>> correct in assuming you get short reads, or rather 0 reads? What file
>>>> system?
>>>
>>> Was going to look into this.
>>> I am getting 0 reads. This is on some old kingston ssd, ext4.
>>
>> I can't seem to reproduce this. I do see some cqe->res == 0 completes,
>> but those appear to be NOPs. And they trigger at the start and end. I'll
>> keep poking.
>
> Nops are used for draining and closing rings at the end of benchmarks.
> It also appears in the beginning because of the way golang runs
> benchmarks...

OK, just checking if it was expected.

But I can reproduce it now, turns out I was running XFS and that doesn't
trigger it. With ext4, I do see zero sized read completions. I'll keep
poking.

-- 
Jens Axboe