Re: [PATCH 0/4] promote zcache from staging

[Seth Jennings <sjenning@linux.vnet.ibm.com>]

On 08/17/2012 05:21 PM, Dan Magenheimer wrote:
>> From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
>> Subject: Re: [PATCH 0/4] promote zcache from staging
>>
>> On 08/09/2012 03:20 PM, Dan Magenheimer wrote
>>> I also wonder if you have anything else unusual in your
>>> test setup, such as a fast swap disk (mine is a partition
>>> on the same rotating disk as source and target of the kernel build,
>>> the default install for a RHEL6 system)?
>>
>> I'm using a normal SATA HDD with two partitions, one for
>> swap and the other an ext3 filesystem with the kernel source.
>>
>>> Or have you disabled cleancache?
>>
>> Yes, I _did_ disable cleancache.  I could see where having
>> cleancache enabled could explain the difference in results.
> 
> Sorry to beat a dead horse, but I meant to report this
> earlier in the week and got tied up by other things.
> 
> I finally got my test scaffold set up earlier this week
> to try to reproduce my "bad" numbers with the RHEL6-ish
> config file.
> 
> I found that with "make -j28" and "make -j32" I experienced
> __DATA CORRUPTION__.  This was repeatable.

I actually hit this for the first time a few hours ago when
I was running performance for your rewrite.  I didn't know
what to make of it yet.  The 24-thread kernel build failed
when both frontswap and cleancache were enabled.

> The type of error led me to believe that the problem was
> due to concurrency of cleancache reclaim.  I did not try
> with cleancache disabled to prove/support this theory
> but it is consistent with the fact that you (Seth) have not
> seen a similar problem and has disabled cleancache.
> 
> While this problem is most likely in my code and I am
> suitably chagrined, it re-emphasizes the fact that
> the current zcache in staging is 20-month old "demo"
> code.  The proposed new zcache codebase handles concurrency
> much more effectively.

I imagine this can be solved without rewriting the entire
codebase.  If your new code contains a fix for this, can we
just pull it as a single patch?

Seth

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