linux-kernel - Re: [PATCH] nvme_core: scan namespaces asynchronously

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Date: Wed, 17 Jan 2024 16:15:53 +0200
From: Sagi Grimberg <sagi@...mberg.me>
To: stuart hayes <stuart.w.hayes@...il.com>,
 Max Gurtovoy <mgurtovoy@...dia.com>, Keith Busch <kbusch@...nel.org>
Cc: linux-kernel@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
 Christoph Hellwig <hch@....de>, linux-nvme@...ts.infradead.org
Subject: Re: [PATCH] nvme_core: scan namespaces asynchronously



On 1/16/24 21:14, stuart hayes wrote:
> 
> 
> On 1/12/2024 1:36 PM, stuart hayes wrote:
>>
>>>
>>>
>>> On 04/01/2024 18:47, Keith Busch wrote:
>>>> On Thu, Jan 04, 2024 at 10:38:26AM -0600, Stuart Hayes wrote:
>>>>> Currently NVME namespaces are scanned serially, so it can take a 
>>>>> long time
>>>>> for all of a controller's namespaces to become available, 
>>>>> especially with a
>>>>> slower (fabrics) interface with large number (~1000) of namespaces.
>>>>>
>>>>> Use async function calls to make namespace scanning happen in 
>>>>> parallel,
>>>>> and add a (boolean) module parameter "async_ns_scan" to enable this.
>>>>
>>>> Hm, we're not doing a whole lot of blocking IO to bring up a namespace,
>>>> so I'm a little surprised it makes a noticable difference. How much 
>>>> time
>>>> improvement are you observing by parallelizing the scan? Is there a
>>>> tipping point in Number of Namespaces where inline scanning is better
>>>> than asynchronous? And if it is a meaningful gain, let's not introduce
>>>> another module parameter to disable it.
>>>
>>> I don't think it is a good idea since some of the namespace 
>>> characteristics must be validated during re-connection time for example.
>>> I actually prepared a patch that makes sure we sync the ns scanning 
>>> before kicking the ns blk queue to avoid that situations.
>>> for example, if for some reason ns1 change its uuid then we must 
>>> remove it and open a new bdev instead. We can't kick old request to 
>>> it...
>>>
>>
>>
>> Sorry for the delayed response--I thought I could get exact data on 
>> how long it takes with and
>> without the patch before I responded, it is taking a while (I'm having 
>> to rely on someone else
>> to do the testing).  I'll respond with the data as soon as I get 
>> it--hopefully it won't be too
>> much longer.  The time it takes to scan namespaces adds up when there 
>> are 1000 namespaces and
>> you have a fabrics controller on a network that isn't too fast.
>>
>> I don't expect there would be any reason to disable this.  I only put 
>> the module parameter to
>> disable it in case there was some unforeseen issue, but I can remove 
>> that.
>>
>> To Max Gurtovoy--this patch wouldn't change when or how namespaces are 
>> validated... it just
>> puts the actual scan work function on a workqueue so the scans can 
>> happen in parallel.  It will
>> do the same work to scan, at the same point, and it will wait for all 
>> the scanning to finish
>> before proceeding.  I don't understand how this patch would make the 
>> situation you mention any
>> worse.
>>
> 
> I have numbers for the namespace scan time improvement.  Below is the 
> amount of time it took for
> all of the namespaces to show up when connecting to a controller with 
> 1002 namespaces:
> 
> network latency   time without patch    time with patch
>    0                        6s                 1s
>   50                      210s                10s
> 100                      417s                18s


That is a big improvement. I wouldn't say that 1000+ namespaces
is a common requirement. But the improvement speaks for itself.