Re: Performance regression in scsi sequential throughput (iozone)due to "e084b - page-allocator: preserve PFN ordering when __GFP_COLD isset"

[Christian Ehrhardt]

Mel Gorman wrote:
[...]
> > Unfortunately even now knowing the place of the issue so well I don't see
> > the connection to the commits e084b+5f8dcc21
>
> Still a mystery.
>
> >  - I couldn't find something but
> > did they change the accounting somewhere or e.g. changed the timing/order
> > of watermark updates and allocations?
>
> Not that I can think of.
>
> > Eventually it might come down to a discussion of allocation priorities and
> > we might even keep them as is and accept this issue - I still would prefer
> > a good second chance implementation, other page cache allocation flags or
> > something else that explicitly solves this issue.
>
> In that line, the patch that replaced congestion_wait() with a waitqueue
> makes some sense.

[...]

> I'll need to do a number of tests before I can move that upstream but I
> don't think it's a merge candidate. Unfortunately, I'll be offline for a
> week starting tomorrow so I won't be able to do the testing.
>
> When I get back, I'll revisit those patches with the view to pushing
> them upstream. I hate to treat symptoms here without knowing the
> underlying problem but this has been spinning in circles for ages with
> little forward progress :(

I'll continue with some debugging in search for the real reasons, but if 
I can't find a new way to look at it I think we have to drop it for now.

While I hate fixing symptoms too, I completely agree that  it is time to 
bring this fix upstream and in fact into the stable kernel too, to have 
at least a ~98% workaround out there.

I'm looking forward for your revised patch after you are back and I'm 
eager to test this one again.


--

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance
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