Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP_SYNC capability

Jens Axboe
2017-08-16T15:56:18Z

On 08/15/2017 10:48 PM, Minchan Kim wrote:
> Hi Jens,
>
> On Mon, Aug 14, 2017 at 10:17:09AM -0600, Jens Axboe wrote:
>> On 08/14/2017 09:38 AM, Jens Axboe wrote:
>>> On 08/14/2017 09:31 AM, Minchan Kim wrote:
>>>>> Secondly, generally you don't have slow devices and fast devices
>>>>> intermingled when running workloads. That's the rare case.
>>>>
>>>> Not true. zRam is really popular swap for embedded devices where
>>>> one of low cost product has a really poor slow nand compared to
>>>> lz4/lzo [de]comression.
>>>
>>> I guess that's true for some cases. But as I said earlier, the recycling
>>> really doesn't care about this at all. They can happily coexist, and not
>>> step on each others toes.
>>
>> Dusted it off, result is here against -rc5:
>>
>> http://git.kernel.dk/cgit/linux-block/log/?h=cpu-alloc-cache
>>
>> I'd like to split the amount of units we cache and the amount of units
>> we free, right now they are both CPU_ALLOC_CACHE_SIZE. This means that
>> once we hit that count, we free all of the, and then store the one we
>> were asked to free. That always keeps 1 local, but maybe it'd make more
>> sense to cache just free CPU_ALLOC_CACHE_SIZE/2 (or something like that)
>> so that we retain more than 1 per cpu in case and app preempts when
>> sleeping for IO and the new task on that CPU then issues IO as well.
>> Probably minor.
>>
>> Ran a quick test on nullb0 with 32 sync readers. The test was O_DIRECT
>> on the block device, so I disabled the __blkdev_direct_IO_simple()
>> bypass. With the above branch, we get ~18.0M IOPS, and without we get
>> ~14M IOPS. Both ran with iostats disabled, to avoid any interference
>> from that.
>
> Looks promising.
> If recycling bio works well enough, I think we don't need to introduce
> new split in the path for on-stack bio.
> I will test your version on zram-swap!

Thanks, let me know how it goes. It's quite possible that we'll need
a few further tweaks, but at least the basis should be there.

--
Jens Axboe