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From: Chris Mason <clm@fb.com>
To: David Sterba <dsterba@suse.cz>
Cc: Josef Bacik <josef@toxicpanda.com>,
	Nikolay Borisov <nborisov@suse.com>,
	"linux-btrfs@vger.kernel.org" <linux-btrfs@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH 2/3] btrfs: wakeup cleaner thread when adding delayed iput
Date: Wed, 28 Nov 2018 19:32:23 +0000
Message-ID: <01FF398F-7C30-4BAD-8562-D4A39589D2F1@fb.com> (raw)
In-Reply-To: <20181128190633.GZ2842@twin.jikos.cz>

On 28 Nov 2018, at 14:06, David Sterba wrote:

> On Tue, Nov 27, 2018 at 03:08:08PM -0500, Josef Bacik wrote:
>> On Tue, Nov 27, 2018 at 07:59:42PM +0000, Chris Mason wrote:
>>> On 27 Nov 2018, at 14:54, Josef Bacik wrote:
>>>
>>>> On Tue, Nov 27, 2018 at 10:26:15AM +0200, Nikolay Borisov wrote:
>>>>>
>>>>>
>>>>> On 21.11.18 г. 21:09 ч., Josef Bacik wrote:
>>>>>> The cleaner thread usually takes care of delayed iputs, with the
>>>>>> exception of the btrfs_end_transaction_throttle path.  The 
>>>>>> cleaner
>>>>>> thread only gets woken up every 30 seconds, so instead wake it up 
>>>>>> to
>>>>>> do
>>>>>> it's work so that we can free up that space as quickly as 
>>>>>> possible.
>>>>>
>>>>> Have you done any measurements how this affects the overall 
>>>>> system. I
>>>>> suspect this introduces a lot of noise since now we are going to 
>>>>> be
>>>>> doing a thread wakeup on every iput, does this give a chance to 
>>>>> have
>>>>> nice, large batches of iputs that  the cost of wake up can be
>>>>> amortized
>>>>> across?
>>>>
>>>> I ran the whole patchset with our A/B testing stuff and the 
>>>> patchset
>>>> was a 5%
>>>> win overall, so I'm inclined to think it's fine.  Thanks,
>>>
>>> It's a good point though, a delayed wakeup may be less overhead.
>>
>> Sure, but how do we go about that without it sometimes messing up?  
>> In practice
>> the only time we're doing this is at the end of finish_ordered_io, so 
>> likely to
>> not be a constant stream.  I suppose since we have places where we 
>> force it to
>> run that we don't really need this.  IDK, I'm fine with dropping it.  
>> Thanks,
>
> The transaction thread wakes up cleaner periodically (commit interval,
> 30s by default), so the time to process iputs is not unbounded.
>
> I have the same concerns as Nikolay, coupling the wakeup to all 
> delayed
> iputs could result in smaller batches. But some of the callers of
> btrfs_add_delayed_iput might benefit from the extra wakeup, like
> btrfs_remove_block_group, so I don't want to leave the idea yet.

Yeah, I love the idea, I'm just worried about a tiny bit of rate 
limiting.  Since we're only waking up a fixed process and not creating 
new work queue items every time, it's probably fine, but a delay of HZ/2 
would probably be even better.

-chris

  reply index

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-11-21 19:09 [PATCH 0/3] Delayed iput fixes Josef Bacik
2018-11-21 19:09 ` [PATCH 1/3] btrfs: run delayed iputs before committing Josef Bacik
2018-11-26 14:44   ` Nikolay Borisov
2018-11-21 19:09 ` [PATCH 2/3] btrfs: wakeup cleaner thread when adding delayed iput Josef Bacik
2018-11-27  8:26   ` Nikolay Borisov
2018-11-27 19:54     ` Josef Bacik
2018-11-27 19:59       ` Chris Mason
2018-11-27 20:08         ` Josef Bacik
2018-11-28 19:06           ` David Sterba
2018-11-28 19:32             ` Chris Mason [this message]
2018-11-28 20:08             ` Filipe Manana
2018-11-29  0:30               ` Qu Wenruo
2018-11-21 19:09 ` [PATCH 3/3] btrfs: replace cleaner_delayed_iput_mutex with a waitqueue Josef Bacik
2018-11-27  8:29   ` Nikolay Borisov
2018-11-27 20:01     ` Josef Bacik
2018-12-03 16:06 [PATCH 0/3][V2] Delayed iput fixes Josef Bacik
2018-12-03 16:06 ` [PATCH 2/3] btrfs: wakeup cleaner thread when adding delayed iput Josef Bacik
2018-12-04  9:21   ` Nikolay Borisov
2018-12-04 18:18     ` Josef Bacik
2019-01-11 15:21 [PATCH 0/3][V3] Delayed iput fixes Josef Bacik
2019-01-11 15:21 ` [PATCH 2/3] btrfs: wakeup cleaner thread when adding delayed iput Josef Bacik

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