The Linux kernel is the operating system kernel used by the Linux family of Unix-like operating systems. It is one of the most prominent examples of free and open source software.

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Intentionally cause an I/O error in Linux?

Is there anyway, with Linux, to purposely cause a block device to report an I/O error, or possibly simulate one for testing purposes?
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9answers
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Maximum number of files in one ext3 directory while still getting acceptable performance?

I have an application writing to an ext3 directory which over time has grown to roughly three million files. Needless to say, reading the file listing of this directory is unbearably slow. I don't ...
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How do I make Linux recognize a new SATA /dev/sda drive I hot swapped in without rebooting?

Hot swapping out a failed SATA /dev/sda drive worked fine, but when I went to swap in a new drive, it wasn't recognized: [root@fs-2 ~]# tail -18 /var/log/messages May 5 16:54:35 fs-2 kernel: ata1: ...
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2answers
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Effects of configuring vm.overcommit_memory

My VPS web server running on CentOS 5.4 (Linux kernel 2.6.16.33-xenU) irregularly (like once a month give or take a few weeks) becomes unresponsive due to oom-killer kicking in. Monitoring of the ...
17
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2answers
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Why CPU spent time on IO(wa)?

I know wa (in top) measures the CPU time on waiting for I/O. Many articles say that. But I am confused that, based on 2 knowledge points: if a process uses a system call to read disk, the process ...
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3answers
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Linux memory fragmentation

Is there a way to detect memory fragmentation on linux ? This is because on some long running servers I have noticed performance degradation and only after I restart process I see better performance. ...
13
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1answer
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Cause of page fragmentation on “large” server with xfs, 20 disks and Ceph

Any insight from someone with a bit of experience in the linux IO system would be helpful. Here is my story: Recently brought up a cluster of six Dell PowerEdge rx720xds to serve files via Ceph. ...
12
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4answers
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In CentOS 6.x, how can I upgrade to Kernel 3.4?

I have a server running CentOS 6.2 with kernel version 2.6.32, but I need to increase my application Performance. The Kernel Version 3.4 has x32abi which can improve the performance so i want to ...
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3answers
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Why does /proc/net/tcp6 represents ::1 as ::100:0

I was writing an utility to check /proc/net/tcp and tcp6 for active connections as its faster than parsing netstat output. As I dont actually have ipv6 enabled I was mainly utilizing localhost as my ...
11
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3answers
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1000 HZ linux kernel necessary if I have tickless and high resolution timer?

I am trying to improve performance on my server. I have a few processes that need low jitter (less than 10ms variance). I have a load average of 4 maximum on an i7-920 (4 physical cores, 8 with HT). ...
11
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5answers
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tc u32 — how to match L2 protocols in recent kernels?

I have a nice shaper, with hashed filtering, built at a linux bridge. In short, br0 connects external and internal physical interfaces, VLAN tagged packets are bridged "transparently" (I mean, no VLAN ...
11
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2answers
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Linux kernel detecting wrong processor frequency

After a cold boot of a 6.0.8 Debian server (HP ProLiant), ntpd played havoc with system time: offset and jitter with respect to the usual and reliable reference time servers growing without limit. (...
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2answers
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How can I tune the initial TCP retransmit timeout?

The initial TCP RTO value of 3s is too long for most LAN-based applications. How can I tune it lower? Is there a sysctl?
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2answers
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Linux Kernel Versioning: Debian Sid vs Ubuntu Precise

I have setup two machines: On one machine I have installed Ubuntu Precise 12.04 beta and dist-upgraded to the latest packages. On the other machine I have installed Debian Sid Unstable and dist-...
9
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1answer
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Flush processes consume too much of CPU

The Server is EC2 Instance, it stands for saving files to NAS (NFS) from HTTPD. The processes like flush-0:32 consume over %90 of CPU and load average: 65.50, 64.02, 66.59. According the graph, it's ...
8
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3answers
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How can I see logs in a server after a kernel panic hang?

I am running a production gentoo Linux machine, and recently there was a situation where the server hung in my co-located premises and when I got there I noticed that the server was hung on what ...
7
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5answers
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Convey multicast (to another LAN) through WAN disabling multicast

My Server and Client softwares run both on Linux. The Server broadcasts multicast, the Client listens multicast. My Client has to migrate to another site, and unfortunately multicast is not allowed ...
7
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2answers
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How do I set the default kernel parameters in CentOS for all existing and future kernels?

I'd like to remove the rhgb and quiet kernel parameters which are used by default when the kernel is booted in CentOS 6, but I want this to apply to all currently installed kernels as well as any ...
7
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1answer
343 views

Difference between net.core.rmem_max and net.ipv4.tcp_rmem

What's the difference between net.core.rmem_max and the third value of net.ipv4.tcp_rmem? Which has the higher priority for tcp connections? For below two examples, what's the max buffer for tcp ...
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5answers
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MySQL 5.5 degraded performance in linux kernel 3.2 compared to 2.6

Our database servers (mainly based on the Debian stable packages (=currently Wheezy) seem to have about 4 times more load for the same workload in kernel 3.2.0-4-amd64 then in it's previous 2.6.32-5-...
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0answers
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Multithreading with multi-queue NIC on SMP system [closed]

How are packets scheduled from network interface queues to CPUs, then onwards to threads for processing? What needs to be considered when it comes to how packets are hashed across queues, hardware ...
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2answers
760 views

Kernel updates without rebooting

Similar to some of the topics touched on in this question, Rebooting servers for kernel updates, I was curious if there was a way in which one could apply kernel updates to the system without ...
6
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2answers
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Linux: echo 3 > /proc/sys/vm/drop_caches takes hours to complete

I have a Thecus N8900 NAS, which is a Linux based file server, providing files via NFS to six clients. For some reason that Thecus support has yet to explain, it runs a script that checks /proc/...
5
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1answer
401 views

Why does strace return so many execve calls with ENOENT return code?

I wanted to use strace to see what was happening when I do `hadoop fs -ls ', so I ran the command: strace -f -e trace=execve hadoop fs -ls /stg which resulted in a lot of output like this: [pid ...
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2answers
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Linux (non-transparent) per-process hugepage accounting

I've recently converted some java apps to run with linux manually-configured hugepages, as described here. I point out "manually-configured" because they are not transparent hugepages, which gave us ...
5
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2answers
933 views

Linux: Why does the CPU frequency fluctuate when using the performance governor?

I'm using a Debian 8 amd64 machine for benchmarking. During experimentation, I would like the CPU to operate at a fixed frequency (preferably the maximum possible). This will rule out the CPU clock ...
5
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2answers
229 views

View full program command line arguments in OOM killer logs

Is it possible to view full program command line arguments in OOM killler logs ? What I see now in /var/log/syslog is Memory cgroup out of memory: Kill process 29187 (beam.smp) score 998 or sacrifice ...
5
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1answer
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why is kswapd using high CPU on an idle system?

I have an idle Linux centOS system and yet kswapd is using 100% cpu. All I have running is a single bash session with top running.... I have 32G RAM and yet kswapd is constantly using 100% cpu for ...
5
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2answers
364 views

Please explain the following o/p of “top” command

Please explain the following line from the "top" command in the bold : Mem: 1026228k total, 449640k used, 576588k free, 44812k buffers Swap: 1052248k total, 2020k used, ...
5
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1answer
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linux initcwnd and initrwnd via /etc/sysctl.conf

Folks, As we know, the 3.x kernels have higher TCP default connection values, such as initcwnd and initrwnd. For 2.x friends, is there a way to set these in /etc/sysctl.conf instead of the ip route ...
5
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1answer
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Linux buffer cache effect on IO writes?

I'm copying large files (3 x 30G) between 2 filesystems on a Linux server (kernel 2.6.37, 16 cores, 32G RAM) and I'm getting poor performance. I suspect that the usage of the buffer cache is killing ...
4
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1answer
269 views

CVE-2014-0196 Vulnerability for CentOS

Trying to patch CVE-2014-0196 Vulnerability but yum can't find packages updates CentOS 6.5 (Final) Panel version: 11.0.9 Update #61 # uname -r 2.6.32-358.11.1.el6.x86_64 # yum update kernel Loaded ...
4
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2answers
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What is mdev and how to /dev/* nodes get created in Linux?

Our Linux kernel is 2.6.31 running on an embedded MIPS processor with busybox. The initialisation script (rcS) starts mdev with: echo "/sbin/stbhotplug" > /proc/sys/kernel/hotplug mdev -s Can ...
4
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3answers
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kswapd often uses 100% CPU when swap is in use

Most of the time when my computer starts to need swap, I see a massive spike in CPU usage (kswapd0 is consistently using 99%-100% CPU). According to top, the time is spent in sy (system/kernel) not wa ...
4
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1answer
4k views

Our embedded linux system won't recognize a USB Device if it is plugged in before powerup. Suggestions?

We are developing on a small embedded device. This device us a gumstix overo board running OpenEmbedded linux. We have our development almost completely done, and have run into the strangest of bugs ...
4
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1answer
159 views

kswap using 100% of CPU even with 100GB+ of RAM available

I'm running a Centos 7 ESXi VM with almost 300GB of RAM and 24 vCPUs. The average load is 3 and apps almost never use more than 150GB of RAM. The rest of the available memory is used by Linux for ...
4
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1answer
775 views

Meaning of tcp_delack_min

the current Linux Kernel (e.g. 2.6.36) uses Delayed Acknowledgments (delack). In /include/net/tcp.h it says: define TCP_DELACK_MIN ((unsigned)(HZ/25)) So, for a Kernel using a HZ value of ...
4
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1answer
36 views

Large percentage of identical systems hang on soft reboot

Reaching out for any other ideas after banging my head against this problem for a week. We have approximately 100 identical systems using the AAEON PICO-ITX BT01 motherboard with J1900 Celeron ...
3
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2answers
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Can't update linux kernel on VPS

I recently started using VPS from OVH: http://www.ovh.co.uk/vps/vps-classic.xml This is likely problem very specific to this one provider. My goal is to install and run Docker on it, for this I need ...
3
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1answer
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Cannot find network subsystem in cgroup on Ubuntu 12.04 LTS

I'm trying to use cgroups to limit a process's network bandwidth, as described in this answer. However, I am unable to find the "files" they refer to. I am using Ubuntu 12.04 LTS with cgroup-bin ...
3
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4answers
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is /dev/dm-1 a partition or a whole block device?

Is there a way to figure out if /dev/dm-1 is a block device partition or a whole block device? If /dev/dm-1 is a partition, is there a way to find out the path to the corresponding whole block device?
3
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1answer
214 views

How can I tell the kernel to free cached memory during memory allocation request instead of the OOM killer terminating the application

I am running Ubuntu 10.04 with the below kernel version Linux ip-10-0-1-119 2.6.32-343-ec2 #45-Ubuntu SMP Tue Feb 14 18:18:17 UTC 2012 x86_64 GNU/Linux The memory status shows that the cache is ...
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3answers
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PostgreSQL won't start because it “Cannot allocate memory”

I'm running PostgreSQL 8.4.5 on Ubuntu 10.04. I run an EC2 instance with a few sites running on nginx. Most of these sites run on Django and connect to this Postgres instance. For some reason, at 8:...
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1answer
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how do i set hard and soft file limits for a non-root user at boot?

I have a xen HVM vps and I'm trying to raise the hard and soft file limits for nobody at boot time. I'm using Ubuntu 10.04.3 and inside /etc/security/limits.conf I have: nobody soft nofile ...
3
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1answer
127 views

How to force logging all terminal actions on Ubuntu 12.04 server?

I believe my server has just got compromised interactively. As this server will go to thrash anyway, I wonder if I can get a better insight of what the villain is doing by forcing logging of all tty....
3
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1answer
538 views

Installed 2TB disk in ubuntu server, dmesg reports it as 9444732965540666 MB

I've have an ageing Linux server running on Dell PowerEdge hardware, with a SAS 5IR RAID controller. It has a pair of SAS 146Gb disks in a RAID 1 configuration that has the OS and other software. I ...
3
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1answer
135 views

Nginx uses both writev() and sendfile()

I noticed that when serving images Nginx is sending images using both writev() when called from a browser such as Firefox, but uses sendfile() to send the image when wgetting the image. Any idea why ...
3
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1answer
126 views

Unidentifiable processes under Redhat Linux

Upon issuing a ps ax command on a server I see the following processes that I can't identify. Googling them yields nothing useful. Does anybody know what these are or how I would track them down? The ...
3
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1answer
209 views

What is the interrest of playing with drop_caches?

I've seen sysadmins playing with /proc/sys/vm/drop_caches (echo 3 into this file from a cron) in order to free caches. Is there a real interrest in doing this? Aren't the clean caches reclaimed when ...
3
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1answer
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Linux kernel crash - any ideas?

Been getting these runaway conditions on a lightly loaded vps (debian squeeze kernel 2.6.38-bpo.2-amd64, 2GB, SSD) - top load is nothing , then jumps to 30.0 & everything grinds to a halt. I'm ...