UBC (Unified Buffer Cache), purge, performance

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UBC (Unified Buffer Cache), purge, performance

Post by grahamperrin » Tue Mar 05, 2013 1:58 pm

Traditionally I advise people to not purge for performance purposes. This advice, because the virtues of inactive and free memory – blue and green in the pie chart presented by Activity Monitor – are often misunderstood.

(In simple terms: yes, a purge might increase the percentage of free memory; but no, you should not assume that overall performance of the Mac will improve. (There may be reduced performance whilst part of the UBC is re-populated with data that must be read from a relatively slow disk.))

Two recent posts make me reconsider the value of purge in ZEVO/ZFS environments:


Thoughts?

Thanks …
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Re: UBC (Unified Buffer Cache), purge, performance

Post by raattgift » Tue Mar 05, 2013 4:10 pm

Purge is really only useful diagnostically. If it produces even a slight quantitative performance boost, then there is an underlying problem that should be found and fixed. If it produces a *large* qualitative difference, that should be quantified to try to prove that running it isn't just a placebo. Almost all of the time it should produce the opposite of a performance boost, since hot filesystem blocks will have to be retrieved from underlying storage (which, for the purposes of the zevo zfs port includes ARC).

Instrumentation should be done with dtrace (e.g. hfsslower.d and other scripts described here http://dtrace.org/blogs/brendan/2011/10 ... -mac-os-x/ ).
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Re: UBC (Unified Buffer Cache), purge, performance

Post by grahamperrin » Wed Mar 06, 2013 2:00 am

Thanks again.

(That top ten page is a great favourite of mine. http://diigo.com/0luwa for a highlighted view.)
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