In computing, the kernel is the main component of most computer operating systems; it is a bridge between applications and the actual data processing done at the hardware level. The kernel's responsibilities include managing the system's resources (the communication between hardware and software ...

learn more… | top users | synonyms

53
votes
22answers
18k views

rm on a directory with millions of files

Background: physical server, about two years old, 7200-RPM SATA drives connected to a 3Ware RAID card, ext3 FS mounted noatime and data=ordered, not under crazy load, kernel 2.6.18-92.1.22.el5, uptime ...
26
votes
3answers
16k views

Turn off the Linux OOM killer by default?

The OOM killer on Linux wreaks havoc with various applications every so often, and it appears that not much is really done on the kernel development side to improve this. Would it not be better, as a ...
20
votes
6answers
20k views

How do I list loaded Linux module parameter values?

Is there a standard way to list the parameter values of a loaded Linux module? I'm essentially probing for another answer to this Linux kernel module parameters question, because the module I'm ...
19
votes
2answers
11k views

Damaged /vmlinuz and /initrd.img symbolic links after Kernel uninstall

OS: Ubuntu 8.04 LTS Server Edition We just rolled back an kernel update using the following command: sudo apt-get remove linux-image-2.6.24-24-server The uninstallation was successful, but it had ...
19
votes
3answers
42k views

Improving TCP performance over a gigabit network with lots of connections and high traffic of small packets

I’m trying to improve my TCP throughput over a “gigabit network with lots of connections and high traffic of small packets”. My server OS is Ubuntu 11.10 Server 64bit. There are about 50.000 (and ...
17
votes
3answers
60k views

“Possible SYN flooding” in log despite low number of SYN_RECV connections

Recently we had an apache server which was responding very slowly due to SYN flooding. The workaround for this was to enable tcp_syncookies (net.ipv4.tcp_syncookies=1 in /etc/sysctl.conf). I posted a ...
16
votes
2answers
2k views

What is the largest hardware clock update the Linux kernel “11-minute mode” can make?

When certain time-related programs (like ntpd) are running on a Linux system, the kernel will switch into so-called "eleven minute mode" (see the hwclock man page) whereby it will automatically update ...
15
votes
6answers
2k views

is ksplice production ready?

I would be interested to hear the serverfault community's experiences with Ksplice in production. Quick blurb from wikipedia: Ksplice is a free and open source extension of the Linux kernel ...
13
votes
3answers
39k views

How to fix “BUG: soft lockup - CPU#0 stuck for 17163091968s”?

UPDATE: I updated the title of the message, because I've recently seen more of these problems with this exact time amount of 17163091968s. This should help people investigating the symptoms to find ...
13
votes
1answer
157 views

Does the “bs” option in “dd” really improve the speed?

Every now and then, I'm told that to increase the speed of a "dd" I should carefully choose a proper "block size". Even here, on ServerFault, someone else wrote that "...the optimum block size is ...
12
votes
8answers
3k views

Is it important to reboot Linux after a kernel update?

I have a few production Fedora and Debian webservers that host our sites as well as user shell accounts (used for git vcs work, some screen+irssi sessions, etc). Occasionally a new kernel update will ...
12
votes
2answers
1k views

Kernel stack trace to source code lines

Given a kernel stack trace as below, how do you determine the specific line of code where the issue occurred? kernel: [<ffffffff80009a14>] __link_path_walk+0x173/0xfb9 kernel: ...
12
votes
2answers
4k views

Strange: why does linux respond to ping with ARP request after last ping reply?

I (and a colleague) have just noticed, and tested, that when a Linux machine is pinged, after the last ping it initiates a unicast ARP request to the machine that initiated the ICMP ping. When pinging ...
11
votes
3answers
22k views

Soft limit vs Hard limit?

Can anyone explain in layman's terms what the difference between soft and hard limit is? Should I set my soft and hard limit to be the same? Or should soft be significantly lower? Does the system ...
10
votes
2answers
8k views

How do ulimit -n and /proc/sys/fs/file-max differ?

I notice that on a new CentOS image that I just booted up off of EC2 that the ulimit default is 1024 open files, but /proc/sys/fs/file-max is set at 761,408 and I'm wondering how these two limits work ...
9
votes
4answers
4k views

How can I troubleshoot high Kernel time?

I have unusually high Kernel time on my CPUs as shown in task manager. What are some ways I can troubleshoot this?
9
votes
2answers
6k views

How do I check what kernel options were compiled without looking at /boot/config-*

How do I check what options are compiled into a linux kernel if I don't have access to the /boot/config-* file that's left over?
9
votes
1answer
2k views

How does vm.overcommit_memory work?

When I use the default settings: vm.overcommit_memory = 0 vm.overcommit_ratio = 50 I can read these values from /proc/meminfo file: CommitLimit: 2609604 kB Committed_AS: 1579976 kB But ...
8
votes
2answers
466 views

Why “lsmod | wc -l” not equal “ls /sys/module | wc -l”

Why do these two commands return different results? lsmod | wc -l 100 ls /sys/module | wc -l 138
8
votes
2answers
3k views

Converting an EC2 AMI to vmdk image

I've come quite close to getting Amazon Linux to boot inside VirtualBox, thanks to this answer and these websites. A quick overview of the steps I've taken: Launch EC2 instance with Amazon Linux ...
8
votes
2answers
831 views

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 ...
8
votes
1answer
392 views

TCP packet being retransmitted 7 times when sysctl tcp_retries1 is set to 3 - why?

Ubuntu 12.04 I am trying to better understand how many times TCP will attempt to retransmit a packet when it does not receive confirmation the destination received it. After reading the tcp man page ...
7
votes
4answers
8k views

Automatically answer defaults when doing 'make oldconfig' on a kernel tree

When building a new kernel based on a previous config, is there a way to automate the 'make oldconfig' process so that it sets new options to their default values? Edit: What I mean is that when ...
7
votes
2answers
606 views

/dev/shm & /proc hardening

I've seen mention of securing /dev/shm and /proc and I was wondering how you do that and what it consists of doing? I assume this involves /etc/sysctl.conf editing of some kind right. Like these? ...
7
votes
2answers
2k views

How can I take advantage of IW10 in kernel 2.6.33?

I've read that 2.6.33+ allows setting custom cwnd. if the IW is 10 by default (for all distros? only some?) how does one view what the current IW is on a particular compiled kernel? references: ...
7
votes
3answers
461 views

Enable SMP on Debian i386?

I've been running a couple old HP machines on Debian for a while, and only recently noticed that they were only 'recognizing' and using one processor. cat /proc/cpuinfo only shows output for processor ...
6
votes
6answers
465 views

How to Minimize Linux Server Reboots

Last week there were a fair few comments on a slashdot article about whether Unix (or Linux) machines ever need to be rebooted. More than a few of the commenters mentioned having machines with ...
6
votes
2answers
610 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
votes
1answer
136 views

RHEL6 Keeping 1 old kernel and security

I have my RHEL 5&6 servers set to keep 1 old kernel after updates, I do this in case I need to recover the system for any number of reasons. My question is related to security of the system when ...
6
votes
1answer
3k views

Why OOM-killer invoked even when huge amount of RAM is free

On EC2 ebs-backed xlarge ubuntu instance, oom-killer is getting invoked. From /var/log/syslog output below, it appears that ZONE_NORMAL is running out of memory: Node 0 Normal free:11344kB ...
6
votes
2answers
1k views

TCP SYN Flooding Detection Method in the Linux Kernel

When the Linux kernel detects SYN Flooding it logs a message like: possible SYN flooding on port 80. Sending cookies Does anyone know the exact method that the kernel uses to detect this?
6
votes
3answers
7k views

How does Linux determine the SCSI address of a disk?

Greetings, I'm working with RHEL 5.5 guest VMs under VMware ESX 4. When I configure the virtual disks in the VM hardware settings, each disk has a SCSI address in the format "N:M". For example, "1:3" ...
6
votes
2answers
1k views

Monolithic vs Micro kernel

What are the differences between a monolithic kernel and a microkernel with respect to structure and security. My friend told me that Linux systems have monolithic kernel and thus are not easier to ...
6
votes
1answer
1k views

Linux arp cache timeout values

I'm trying to configure sane values for the Linux kernel arp cache timeout, but I can't find a detailed explanation as to how they work anywhere. Even the kernel.org documentation doesn't give a good ...
5
votes
4answers
22k views

In CentOS 6.x, how can I upgrade to Kernel 3.4?

I have a server with 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 upgrade ...
5
votes
2answers
13k views

Debian: logging of SSH failed login attempts?

I just typed a wrong password for login to ssh @ root. I went to /var/log/faillog But the file is empty ( tho its filesize is: 32 Byte ) Ok in auth.log is spammed this: reverse mapping checking ...
5
votes
4answers
1k views

The Gentoo live-cd shows my drives as “hda”, but booting into my own kernel shows “sda” (therefore, boot fails). What should I do?

The Gentoo live-cd shows my drives as "hda", so I followed the Gentoo handbook and configured my partitions as "hda". However, the boot failed because, when booting into my kernel, it wanted to refer ...
5
votes
2answers
7k views

How do you check the version of a *.ko kernel module in Linux?

I know you can do: sudo modprobe -v some_module to check the version of /lib/modules/.../some_module.ko, but I'd still like to be able to check the version of any arbitrary kernel module that's not ...
5
votes
4answers
3k views

Prevent the console from clearing the screen?

When booting into Linux, there are sometimes one or two lines that get quickly cleared. I think that some of them don't even appear in dmesg. If nothing else, I want to suppress the clear before the ...
5
votes
2answers
8k views

Can I increase inode count in Linux?

Sorry for my poor English. I have a Linux MIPS router with 2.4.17 kernel. Root fs is a tmpfs and /rom is a cramfs. There is 4MB free memory and 3MB free on tmpfs. I can create a few new empty ...
5
votes
2answers
534 views

Optimal kernel configuration and patches for KVM host

I have a server (single socket Nehalem w/ 24GB RAM) acting primarily as a KVM host containing a bunch of Windows Servers and a few (tickless) Linux instances. I usually compile my desktop kernels ...
5
votes
3answers
22k views

How to set shmall, shmmax, shmni, etc … in general and for postgresql

I've used the documentation from PostgreSQL to set it for example this config: >>> cat /proc/meminfo MemTotal: 16345480 kB MemFree: 1770128 kB Buffers: 382184 kB ...
5
votes
4answers
11k views

Find out which task is generating a lot of context switches on linux

According to vmstat, my Linux server (2xCore2 Duo 2.5 GHz) is constantly doing around 20k context switches per second. # vmstat 3 procs -----------memory---------- ---swap-- -----io---- -system-- ...
5
votes
4answers
6k views

How can I find out what is causing interrupts on Windows?

Occasionally I come across servers (Windows 2003 and 2008) with high processor % interrupt time. Is there a way to see what program or device is causing the interrupts?
5
votes
3answers
5k views

Interpreting cryptic kernel “page allocation failure” messages

I have a multi-user CentOS 6.1 database system running an application named ABC. The server is 64-bit, 8GB RAM and 6 vCPU (on VMWare ESXi 4). We get frequent dumps into the dmesg and the system logs ...
5
votes
3answers
5k views

Kernel Log “TCP: Treason uncloaked!”

On one linux server (Gentoo hardened), we are experiencing bursts of the following messages in dmesg from time to time: TCP: Treason uncloaked! Peer xx.xx.xxx.xxx:65039/80 shrinks window ...
5
votes
1answer
1k views

Why won't FreeBSD reboot after a kernel crash?

Once in a while I get my server running FreeBSD 8.0 amd64 fail due to bad memory (incompatible with motherboard) modules. Each time it happens, the box stalls with the last note saying that it will ...
5
votes
2answers
1k views

Still getting aacraid: Host adapter abort request errors after following recommended steps

I am getting the infamous aacraid: Host adapter abort request error with my new Adaptec RAID Controller under high I/O. I have read several forums, even Adaptecs, that setting the ...
5
votes
1answer
230 views

Linux Kernel lowmem usage

I am debugging an issue where we are running out of kernel low memory and I am trying to identify who is using the kernel low memory thats contributing to the problem. Following is the meminfo ...
4
votes
4answers
1k views

Server randomly freezes

Im facing a very strange issue, my debian squeeze freezes up always at night (Berlin, time). Here is what i get from a time and after doing this a few times, it becomes frozen and must be hard-reset. ...