Re: Tux3 Report: How fast can we fsync?

[Daniel Phillips]

On Friday, May 1, 2015 6:07:48 PM PDT, David Lang wrote:
> On Fri, 1 May 2015, Daniel Phillips wrote:
> > On Friday, May 1, 2015 8:38:55 AM PDT, Dave Chinner wrote:
> > > Well, yes - I never claimed XFS is a general purpose filesystem.  It
> > > is a high performance filesystem. Is is also becoming more relevant
> > > to general purpose systems as low cost storage gains capabilities
> > > that used to be considered the domain of high performance storage...
> >
> > OK. Well, Tux3 is general purpose and that means we care about single
> > spinning disk and small systems.
>
> keep in mind that if you optimize only for the small systems 
> you may not scale as well to the larger ones.

Tux3 is designed to scale, and it will when the time comes. I look 
forward to putting Shardmap through its billion file test in due course. 
However, right now it would be wise to stay focused on basic 
functionality suited to a workstation because volunteer devs tend to 
have those. After that, phones are a natural direction, where hard core 
ACID commit and really smooth file ops are particularly attractive.

> per the ramdisk but, possibly not as relavent as you may think. 
> This is why it's good to test on as many different systems as 
> you can. As you run into different types of performance you can 
> then pick ones to keep and test all the time.

I keep being surprised how well it works for things we never tested 
before.

> Single spinning disk is interesting now, but will be less 
> interesting later. multiple spinning disks in an array of some 
> sort is going to remain very interesting for quite a while.

The way to do md well is to integrate it into the block layer like 
Freebsd does (GEOM) and expose a richer interface for the filesystem. 
That is how I think Tux3 should work with big iron raid. I hope to be
able to tackle that sometime before the stars start winking out.

> now, some things take a lot more work to test than others. 
> Getting time on a system with a high performance, high capacity 
> RAID is hard, but getting hold of an SSD from Fry's is much 
> easier. If it's a budget item, ping me directly and I can donate 
> one for testing (the cost of a drive is within my unallocated 
> budget and using that to improve Linux is worthwhile)

Thanks.

> As I'm reading Dave's comments, he isn't attacking you the way 
> you seem to think he is. He is pointing ot that there are 
> problems with your data, but he's also taking a lot of time to 
> explain what's happening (and yes, some of this is probably 
> because your simple tests with XFS made it look so bad)

I hope the lightening up trend is a trend.

> the other filesystems don't use naive algortihms, they use 
> something more complex, and while your current numbers are 
> interesting, they are only preliminary until you add something 
> to handle fragmentation. That can cause very significant 
> problems.

Fsync is pretty much agnostic to fragmentation, so those results are 
unlikely to change substantially even if we happen to do a lousy job on 
allocation policy, which I naturally consider unlikely. In fact, Tux3 
fsync is going to get faster over time for a couple of reasons: the 
minimum blocks per commit will be reduced, and we will get rid of most 
of the seeks to beginning of volume that we currently suffer per commit.

> Remember how fabulous btrfs looked in the initial 
> reports? and then corner cases were found that caused real 
> problems and as the algorithms have been changed to prevent 
> those corner cases from being so easy to hit, the common case 
> has suffered somewhat. This isn't an attack on Tux2 or btrfs, 
> it's just a reality of programming. If you are not accounting 
> for all the corner cases, everything is easier, and faster.

> > Mine is a lame i5 minitower with 4GB from Fry's. Yours is clearly way
> > more substantial, so I can't compare my numbers directly to yours.
>
> If you are doing tests with a 4G ramdisk on a machine with only 
> 4G of RAM, it seems like you end up testing a lot more than just 
> the filesystem. Testing in such low memory situations can 
> indentify significant issues, but it is questionable as a 'which 
> filesystem is better' benchmark.

A 1.3 GB tmpfs, and sorry, it is 10 GB (the machine next to it is 4G). 
I am careful to ensure the test environment does not have spurious 
memory or cpu hogs. I will not claim that this is the most sterile test 
environment possible, but it is adequate for the task at hand. Nearly 
always, when I find big variations in the test numbers it turns out to 
be a quirk of one filesystem that is not exhibited by the others. 
Everything gets multiple runs and lands in a spreadsheet. Any fishy 
variance is investigated.

By the way, the low variance kings by far are Ext4 and Tux3, and of 
those two, guess which one is more consistent. XFS is usually steady, 
but can get "emotional" with lots of tasks, and Btrfs has regular wild 
mood swings whenever the stars change alignment. And while I'm making 
gross generalizations: XFS and Btrfs go OOM way before Ext4 and Tux3.

> Just a suggestion, but before you do a huge post about how 
> great your filesystem is performing, making the code avaialble 
> so that others can test it when prompted by your post is 
> probably a very good idea. If it means that you have to send out 
> your post a week later, it's a very small cost for the benefit 
> of having other people able to easily try it on hardware that 
> you don't have access to.

Next time. This time I wanted it off my plate as soon as possible so I 
could move on to enospc work. And this way is more involving, we get a 
little suspense before the rematch.

> If there is a reason to post wihtout the code being in the 
> main, publicised repo, then your post should point people at 
> what code they can use to duplicate it.

I could have included the patch in the post, it is small enough. If it 
still isn't in the repo in a few days then I will post it, to avoid 
giving the impression I'm desperately trying to fix obscure bugs in it, 
which isn't the case.

> but really, 11 months without updating the main repo?? This is 
> Open Source development, publish early and often.

It's not as bad as that:

  https://github.com/OGAWAHirofumi/linux-tux3/commits/hirofumi
  https://github.com/OGAWAHirofumi/linux-tux3/commits/hirofumi-user

> something to investigate, but I have seen probelms on ext* in 
> the past. ext4 may have fixed this, or it may just have moved 
> the point where it triggers.

My spectrum of tests is small and I am not hunting for anomalies, only 
reporting what happened to come up. It is not very surprising that some
odd things happen with 10,000 tasks, there is probably not much test 
coverage there. On the whole I was surprised and impressed when all 
filesystems mostly just worked. I was expecting to hit scheduler issues 
for one thing, and nothing obvious came up. Also, not one oops on any 
filesystem (even Tux3) and only one assert, already reported upstream 
and turned out to be fixed a week or two ago.

> > > ...
> > Your machine makes mine look like a PCjr. ...
>
> The interesting thing here is that on the faster machine btrfs 
> didn't speed up significantly while ext4 and xfs did. It will be 
> interesting to see what the results are for tux3

The numbers are well into the something-is-really-wrong zone (and I 
should have flagged that earlier but it was a long day). That test is 
supposed to be -s, all synchronous, and his numbers are more typical of
async. Needs double checking all round, including here. Anybody can 
replicate that test, it is only an apt-get install dbench away (hint 
hint).

Differences: my numbers are kvm with loopback mount on tmpfs. His are 
on ramdisk and probably native. I have to reboot to make a ramdisk big 
enough to run dbench and I would rather not right now.

How important is it to get to the bottom of the variance in test 
results running on RAM? Probably important in the long run, because 
storage devices are looking more like RAM all the time, but as of 
today, maybe not very urgent.

Also, I was half expecting somebody to question the wisdom of running 
benchmarks under KVM instead of native, but nobody did. Just for the 
record, I would respond: running virtual probably accounts for the
majority of server instances today.

> and both of you need to remember that while servers are getting 
> faster, we are also seeing much lower power, weaker servers 
> showing up as well. And while these smaller servers are not 
> trying to do teh 10000 thread fsync workload, they are using 
> flash based storage more frequently than they are spinning rust 
> (frequently through the bottleneck of a SD card) so continuing 
> tests on low end devices is good.

Low end servers and embedded concerns me more, indeed. 

> what drives are available now? see if you can get a couple 
> (either directly or donated)

Right, time to hammer on flash.

Regards,

Daniel
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