Re: How do you detect memory issues ?

Thanks Mladen
"sar -B" works on Amazon Linux
and it still amazes me how non-obvious monitoring memory pressure is to this day

On Wed, Dec 5, 2018 at 10:28 PM Mladen Gogala <gogala.mladen_at_gmail.com> wrote:

> Hi Kyle,
>
> You are talking about vmstat. I prefer sar. Here is the output of sar -B 3
> 3:
>
> mgogala_at_umajor:~$ sar -B 3 3
> Linux 4.15.0-42-generic (umajor) 12/06/2018 _x86_64_ (8 CPU)
>
> 01:10:34 AM pgpgin/s pgpgout/s fault/s majflt/s pgfree/s pgscank/s
> pgscand/s pgsteal/s %vmeff
> 01:10:37 AM 23049.33 3421.33 16.00 0.00 81.33
> 0.00 0.00 0.00 0.00
> 01:10:40 AM 19186.67 1.33 102.67 0.00 116.00
> 0.00 0.00 0.00 0.00
> 01:10:43 AM 14.67 5064.00 32142.67 0.00 25249.00
> 0.00 0.00 0.00 0.00
> Average: 14083.56 2828.89 10753.78 0.00 8482.11
>
>
>
> The important stats are majflts/s, which means that pages had to be read
> from disk and pgsteal/s, which denotes the number of the modified pages
> backed up and reclaimed as "free". In this context "free" doesn't mean
> empty, the page being free means that the page has a valid backup. Page
> stealing definitel and paging out (pgpgout/s) definitely means that there
> is a memory problem. On Red Hat systems, sar is available in the sysstat
> package. Another good indication that something is wrong is large
> proportion of kernel mode cpu time, as shown by top. Also, "top" is a good
> indicator because it shows the swap usage. If the swap usage keeps growing,
> there is a trouble with memory.
> Regards
>
> On 12/5/18 7:44 PM, kyle Hailey wrote:
>
> One of those questions that seems like it should have been nailed down 20
> years ago but it still seems lack a clear answer
>
> How do you detect memory issues ?
>
> I always used "*po" or "paged outs*". Now on Amazon Linux I don't see
> "po" but there is "bo" (blocks written out). In past, at least on OSF &
> Ultrix, page outs were a sign of needed memory that was written out to disk
> and when I needed that memory it would take a big performance hit to read
> it in. Thus "po" was a good canary on the coal mine. Any consistent values
> over over say 10 were a sign.
>
> Some people use "*scan rate*" but I never found that as easy to interpret
> as page outs. Again what values would you use
>
> Some suggest using freeable memory as a yardstick where freeable is
> "free" + "cached" or MemFree + Cached + Inactive. Even in this case what
> would you use for values to alert on?
>
> I've always ignored swap stats as if you are swapping it is too late.
>
> What do you use to detect memory issues ?
>
> Kyle
>
> --
> Mladen Gogala
> Database Consultant
> Tel: (347) 321-1217
>
>

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