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Re: [RFC PATCH 0/4] xfs: parallel quota check

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [RFC PATCH 0/4] xfs: parallel quota check
From: Jeff Liu <jeff.liu@xxxxxxxxxx>
Date: Wed, 13 Nov 2013 10:27:48 +0800
Cc: "xfs@xxxxxxxxxxx" <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <20131112210349.GA6188@dastard>
References: <5281F4EB.9060108@xxxxxxxxxx> <20131112210349.GA6188@dastard>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.1.0
Thanks for your quick response!

On 11/13 2013 05:03 PM, Dave Chinner wrote:
> On Tue, Nov 12, 2013 at 05:29:15PM +0800, Jeff Liu wrote:
>> Hi Folks,
>>
>> We have a user report about skip quota check on first mount/boot several
>> monthes ago, the original discussion thread can be found at:
>> http://oss.sgi.com/archives/xfs/2013-06/msg00170.html.
>>
>> As per Dave's suggestion, it would be possible to perform quota check
>> in parallel, this patch series is just trying to follow up that idea.
>>
>> Sorry for the too long day as I have to spent most of time dealing with
>> personl things in the last few monthes, I was afraid I can not quickly
>> follow up the review procedure.  Now the nightmare is over, it's time to
>> revive this task.
>>
>> Also, my previous test results on my laptop and a poor desktop can not
>> convience me that performs parallism quota check can really get benefits
>> compare to the current single thread as both machines are shipped with
>> slow disks, I even observed a little performance regression with millions
>> of small files(e.g, 100 bytes) as quota check is IO bound, additionaly,
>> it could affected by the seek time differences.  Now with a Mackbook Air
>> I bought recently, it can show significant difference.
> 
> Results look good - they definitely point out that we can improve
> the situation here.
> 
>> In order to get some more reasonable results, I ask a friend helping
>> run this test on a server which were shown as following.
>>
>> test environment
>> - 16core, 25G ram, normal SATA disk, but the XFS is resides on a loop dev. 
> ....
>>
>> In this case, there is no regression although there is no noticeable
>> improvements. :(
> 
> Which is no surprise - there isn't any extra IO parallelism that can
> be extracted from a single spindle....
> 
>> test environment
>> - Macbook Air i7-4650U with SSD, 8G ram
>>
>> - # of file(million) default                 patched
>>      1               real 0m6.367s           real 0m1.972s
>>                      user 0m0.008s           user 0m0.000s
>>                      sys  0m2.614s           sys  0m0.008s
>>
>>      2               real 0m15.221s          real 0m3.772s
>>                      user 0m0.000s           user 0m0.000s
>>                      sys  0m6.269s           sys  0m0.007s
>>
>>      5               real 0m36.036s          real 0m8.902s
>>                      user 0m0.000s           user 0m0.002s
>>                      sys  0m14.025s          sys  0m0.006s
> 
> But a SSD or large raid array does have unused IO parallelism we can
> exploit. ;)
> 
> Note that there is also the possibility of applying too much
> parallelism for the underlying storage (think of a filesystem with
> hundreds of AGs on a limited number of spindles) and hence causing
> degradations due to seeking. Hence it might be worthwhile to limit
> the number of AGs being scanned concurrently...
Ok, maybe it could be a new mount option to let user decide how to deal
with it in this situation, let me think it over.

> 
>> Btw, The current implementation has a defeat considering the duplicated
>> code at [patch 0/4] xfs: implement parallism quota check at mount time.
>> Maybe it's better to introduce a new function xfs_bulkstat_ag() which can
>> be used to bulkstat inodes per ag, hence it could shared at above patch while
>> adjusting dquota usage per ag, i.e, xfs_qm_dqusage_adjust_perag().
> 
> Right, there are uses for AG-based parallelism of bulkstat for
> userspace, so exposing single AG scans via the bulkstat ioctl is
> something I've been intending to do for some time.  Hence I'd much
> prefer to see xfs_bulkstat_ag() to be implemented and then the
> quotacheck code converted to use it rather than duplicating the
> algorithm and code specifically to parallelise quotacheck.
Thanks for the confirmation, this change will be reflected in the next
round of post.

> 
> I like the factoring of the bulkstat code (about time we did that),
> but I think the factored functions should remain in xfs-itable.c
> with the rest of the bulkstat code for now...
> 
> Also, there's a race condition you haven't handled in the quotacheck
> code: xfs_qm_quotacheck_dqadjust() can now be called concurrently on
> a dquot from different threads to update the same dquot, and there's
> no locking of the dquot to prevent this.
Ah, will fix it, why I have not found this problem in the previous test? :-P
> 
> As to the workqueues for threading, it seems overly complex. You
> could create a permanent workqueue in xfs_init_workqueues() for
> this, and you can use flush_workqueue() to execute and wait for all
> the per-ag scans to complete once they have been queued. This gets
> rid of all the lists and completions from the code.
At that time, I thought the workqueue should be destroyed once the quota
check procedure is complete as it only run once at mount time, will take care
of it.


Thanks,
-Jeff 

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