[ovirt-devel] VDSM changes Linux memory dirty ratios - why?

[Ben England]

I uploaded the article to:

https://s3.amazonaws.com/ben.england/rhev-vm-rsptime.pdf

I had just gone to work for Red Hat back in 2011 so I made the mistake of including a company-internal URL in a public-facing document.    We can update the bz with the above link if you want.  This article is ANCIENT (2011), so while the basic approach should still be relevant, some of the specifics may have changed.  This was a RHEV-on-NFS configuration.

As for why dirty ratio lowering is not part of a tuned profile, there is a debate in the performance team about dirty ratio, with some advocating for higher dirty ratios that help some application workloads (example: writing /tmp files).   I'd stand by this article for the particular configuration that was being studied though, and I think the basic approach of controlling latency and improving fairness by managing queue depths is very relevant today.

cc'ed Sanjay Rao, who continues to be a RHEV expert in the perf team (I no longer am working on it).

-ben


----- Original Message -----
> From: "Martin Polednik" <mpolednik at redhat.com>
> To: "Sven Kieske" <s.kieske at mittwald.de>
> Cc: devel at ovirt.org, "Ben" <bengland at redhat.com>
> Sent: Wednesday, November 30, 2016 7:22:05 AM
> Subject: Re: VDSM changes Linux memory dirty ratios - why?
> 
> On 30/11/16 13:10 +0100, Sven Kieske wrote:
> >On 30/11/16 08:48, Martin Polednik wrote:
> >> It's not really irrelevant, the host still uses disk cache. Anyway,
> >> there is BZ[1] with a presentation[2] that (imho reasonably) states:
> >>
> >> "Reduce dirty page limits in KVM host to allow
> >> direct I/O writer VMs to compete successfully
> >> with buffered writer processes for storage
> >> access"
> >>
> >> I wonder why virtual-host tuned profile doesn't contain these values:
> >>
> >> $ grep vm.dirty /usr/lib/tuned/virtual-host/tuned.conf
> >> vm.dirty_background_ratio = 5
> >>
> >> [1]https://bugzilla.redhat.com/show_bug.cgi?id=740887
> >> [2]http://perf1.lab.bos.redhat.com/bengland/laptop/rhev/rhev-vm-rsptime.pdf
> >
> >Could you share [2] with the wider community? This would be awesome!
> 
> Sorry, I've totally missed the fact it's internal link (unfortunately
> publicly visible on the BZ). I believe it's slightly outdated, but
> let's ask the author.
> 
> Ben, is the document[2] somehow still valid and could it be made publicly
> available?
> 
> Re-referencing for completeness:
> [2]http://perf1.lab.bos.redhat.com/bengland/laptop/rhev/rhev-vm-rsptime.pdf
> 
> >--
> >Mit freundlichen Grüßen / Regards
> >
> >Sven Kieske
> >
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> >
> 
> 
> 
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