linux-kernel - 3TB memory and high system load 80%+

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Date:	Fri, 13 Mar 2015 17:26:43 +0100
From:	jesper@...gh.cc
To:	linux-kernel@...r.kernel.org
Subject: 3TB memory and high system load 80%+ 

Hi.

We have severe issues getting a Intel(R) Xeon(R) CPU E7-8857 v2 @ 3.00GHz
48 cores server to perform nice.

We have just setup a new server for a PostgreSQL database with 3TB of
memory, primarily for disk-io-caching. The server is running Ubuntu 12.04
on kernel 3.13.0-43-generic. The only change is that we have:

echo never > /sys/kernel/mm/transparent_hugepage/enabled

We have "system load spikes" on it. So it occationally are peaking at 80-90%
system load for minutes and then migrating back to "normal". When it happens
all other activity suffers.

Before disabling hugepages, the system pretty much went to a complete
stall when memory pressure occoured.

sar reports
13:05:01        CPU     %user     %nice   %system   %iowait    %steal    
%idle
16:25:01        all     11.20      0.00      2.11      3.75      0.00    
82.94
16:30:01        all     12.55      0.00      2.67      3.63      0.00    
81.15
16:35:01        all     12.24      0.00      3.68      3.61      0.00    
80.47
16:40:01        all     11.74      0.00      2.31      3.46      0.00    
82.48
16:45:01        all     15.12      0.00      4.31      3.85      0.00    
76.71
16:50:01        all     43.04      0.00      8.89      5.24      0.00    
42.82
16:55:01        all     46.14      0.00     13.53      5.29      0.00    
35.03
17:05:01        all     27.24      0.00     40.16      3.77      0.00    
28.83
17:10:01        all     17.24      0.00     57.76      2.68      0.00    
22.32
17:15:01        all     44.89      0.00     11.14      4.44      0.00    
39.53
17:20:03        all     29.73      0.00     33.77      3.99      0.00    
32.51

In above workload it is "only" running pg_dump and pg_restore with 2
instances of postgresql running. No network/nfs/etc at the same time and
not more than 48 processes active at the same time.

Any suggestions/ideas for how/what to improve are greatly welcome. Both in
terms of tunables and potential fixes in kernels.

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