Re: [PATCHSET v5 0/12] Add support for async buffered reads

Jens Axboe
2020-06-05T14:42:34Z

On 6/3/20 7:04 PM, Jens Axboe wrote:
> On 6/3/20 6:59 PM, Andres Freund wrote:
>> Hi,
>>
>> I was trying to benchmark the benefits of this for the io_uring using
>> postgres I am working on. The initial results where quite promising
>> (reducing cpu usage significantly, to lower than non-uring sync io). But
>> unfortunately trying another workload triggered both panics and before
>> that seemingly returned wrong data.
>>
>> I first saw that problem with b360d424ce02, which was
>> linux-block/async-buffered.6 at the time. After hitting the issue, I
>> updated to the current linux-block/async-buffered.6, but the problem
>> persists.
>>
>> The workload that triggers the bug within a few seconds is postgres
>> doing a parallel sequential scan of a large table (and aggregating the
>> data, but that shouldn't matter). In the triggering case that boils down
>> to 9 processes sequentially reading a number of 1GB files (we chunk
>> tables internally into smaller files). Each process will read a 512kB
>> chunk of the file on its own, and then claim the next 512kB from a
>> shared memory location. Most of the IO will be READV requests, reading
>> 16 * 8kB into postgres' buffer pool (which may or may not be neighboring
>> 8kB pages).
>
> I'll try and reproduce this, any chance you have a test case that can
> be run so I don't have to write one from scratch? The more detailed
> instructions the better.

Can you try with async-buffered.7? I've rebased it on a new mechanism,
and doing something like what you describe above I haven't been able
to trigger anything bad. I'd try your test case specifically, so do let
know if it's something I can run.

--
Jens Axboe