Re: [PATCHSET v8 0/12] Uncached buffered IO

[Jens Axboe]

On 1/13/25 5:46 PM, Andrew Morton wrote:
> On Mon, 13 Jan 2025 08:34:18 -0700 Jens Axboe <axboe@xxxxxxxxx> wrote:
> 
>>>
>>
>> ...
>>
>>> Of course, we're doing something here which userspace could itself do:
>>> drop the pagecache after reading it (with appropriate chunk sizing) and
>>> for writes, sync the written area then invalidate it.  Possible
>>> added benefits from using separate threads for this.
>>>
>>> I suggest that diligence requires that we at least justify an in-kernel
>>> approach at this time, please.
>>
>> Conceptually yes. But you'd end up doing extra work to do it. Some of
>> that not so expensive, like system calls, and others more so, like LRU
>> manipulation. Outside of that, I do think it makes sense to expose as a
>> generic thing, rather than require applications needing to kick
>> writeback manually, reclaim manually, etc.
>>
>>> And there's a possible middle-ground implementation where the kernel
>>> itself kicks off threads to do the drop-behind just before the read or
>>> write syscall returns, which will probably be simpler.  Can we please
>>> describe why this also isn't acceptable?
>>
>> That's more of an implementation detail. I didn't test anything like
>> that, though we surely could. If it's better, there's no reason why it
>> can't just be changed to do that. My gut tells me you want the task/CPU
>> that just did the page cache additions to do the pruning to, that should
>> be more efficient than having a kworker or similar do it.
> 
> Well, gut might be wrong ;)

A gut this big is rarely wrong ;-)

> There may be benefit in using different CPUs to perform the dropbehind,
> rather than making the read() caller do this synchronously.
> 
> If I understand correctly, the write() dropbehind is performed at
> interrupt (write completion) time so that's already async.

It does, but we could actually get rid of that, at least when called via
io_uring. From the testing I've done, doing it inline it definitely
superior. Though it will depend on if you care about overall efficiency
or just sheer speed/overhead of the read/write itself.

-- 
Jens Axboe