linux-ext4 - Re: HUGE slowdown when doing dpkg with ext4 over nbd

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Date:   Fri, 9 Dec 2016 13:28:05 -0700
From:   Andreas Dilger <adilger@...ger.ca>
To:     Dave Chinner <david@...morbit.com>
Cc:     Renaud Mariana <rmariana@...ine.net>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        debian-dpkg@...ts.debian.org
Subject: Re: HUGE slowdown when doing dpkg with ext4 over nbd

On Dec 8, 2016, at 6:25 PM, Dave Chinner <david@...morbit.com> wrote:
> 
> On Wed, Dec 07, 2016 at 07:34:17PM +0100, Sven Joachim wrote:
>> On 2016-12-07 11:16 -0700, Andreas Dilger wrote:
>> 
>>> Add debian-dpkg mailing list to CC.
>>> 
>>> On Dec 7, 2016, at 10:58 AM, Andreas Dilger <adilger@...ger.ca> wrote:
>>>> 
>>>> On Dec 7, 2016, at 2:52 AM, Renaud Mariana <rmariana@...ine.net> wrote:
>>>>> 
>>>>> Here are my answers, hope it will help solve this issue, thanks.
>>>>> 
>>>>> Recap:
>>>>> dpkg kibana on ext4 over a nbd device takes 10 minutes
>>>>> with xfs it's only 30s.
>>>>> with ext4 no extends only 30s.
>>>>> 
>>>>> 
>>>>> kernels :
>>>>> 4.5.7 has this issue as older kernel like 4.4.34
>>>>> The issue is also when nbd client & server run on same host
>>>>> 
>>>>> 
>>>>> How small are the files?
>>>>> here is the histogram of file sizes : http://pasteboard.co/6HC3nKyk2.png
>>>>> We can see 5000 files around 512 Bytes.
>>>> 
>>>> Definitely there is no value to use fallocate for 512-byte files, or any
>>>> of the files that can be written in a single write() syscall.  I'd expect
>>>> any reasonable tool to be using a write buffer of at least 2-4MB these
>>>> days to get good performance, so writes below the buffer size shouldn't
>>>> use fallocate() at all.
>> 
>> It should be noted that the latest dpkg (1.18.15) only uses fallocate
>> for files which are at least 16 KiB in size[1], so it would be nice if
>> Renaud could recheck with that version, or cherry-pick the patch into
>> whatever version he uses.
> 
> The fallocate() call should be removed completely. Applications
> should not be attempting to control file allocation like this as it
> defeats all the optimisations that filesystems use to optimise IO
> patterns and minimise filesystem fragmentation (e.g. delayed
> allocation).
> 
> There is /rarely/ a need for applications to use fallocate() to
> manage fragmentation - especailly as excessive use of fallocate()
> actively harms performance and accelerates filesystem aging.
> 
> Unless an application has a specific, repeatable performance problem
> due to file fragmentation, it should not be using fallocate() to
> allocate file space.

I'm not sure I'd go so far as to say that fallocate() should be removed
completely.  Isn't that the best (only) way for an application to tell
the filesystem that it is about to write a file of X size and try to
find a suitable amount of free space for it?  Otherwise, if the file
is large and/or written slowly and/or the system has memory pressure
the filesystem (even with delalloc) can't make a good decision about
allocation.  However, fallocate() won't really help if the file size
is small (e.g. a few MB) since that can easily fit into RAM and will
be written to disk in a single chunk.

Cheers, Andreas






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