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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Unusually high dentry cache usage leads to PHP performance degradation

We have lots of servers serving lots of requests with apache and php accessing remote memcacheds. But we are experiencing degradation from php side accessing memcached and obtaining lots of memcached ...
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2answers
66 views

For php webserver, what is faster, 1x 24-core server or 3x 8-core servers? [closed]

Pricing at Dell, it seems I can either get a single R430 or 3x R220's. The R220's would have 4C/8T processors, giving 24 threads. The R430 would have 2x 6C/12T processors giving again 24 threads. For ...
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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0answers
117 views

Auto-Reboot on Kernel BUG (not tainted, “fixing recursive fault but reboot is needed”)

I'm running a sometimes highly loaded Debian 7 VM with the latest 3.2.x kernel on a XenServer 6.2 host. Sometimes this VM runs into the following state: Ping is okay, All Services unreachable (...
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1answer
145 views

Centos iptables: No chain/target/match by that name [duplicate]

I had a CentOS6.7 with OpenVZ. Now, I just uninstalled OpenVZ and start using Plain CentOS6.7. Due to some difficulties for using cPanel in OpenVZ. I noticed one problem when install csf in my plain ...
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1answer
50 views

Linux OOM-killer disabling woes

I have a Linux machine with no swap, which has vm.overcommit_memory=2 and vm.overcommit_ratio=100 set. However these settings seem to have no effect. Some newly-started processes which attempt to ...
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1answer
130 views

Ubuntu 14.04 TCP established hash table entries

A bit of background I am running two servers under high traffic one with ubuntu 12.04 (linux 3.2.0-69-generic) and one with ubuntu 14.04 (linux 3.13.0-52-generic). I am now trying to secure both. ...
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1answer
41 views

Empty fstab on Debian 7 and up. Possible side effects?

When I was checking a machine set up by our sysadmin team, I encountered an completely empty fstab. In fact it was the default fstab installed by a debootstrap. # UNCONFIGURED FSTAB FOR BASE SYSTEM ...
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2answers
144 views

Hide kernel messages in plymouth

I am running Ubuntu 14.04 server for Kiosk application. The problem is when kiosk is starting , kernel shows some debug messages about stuff like USB port error and such things, and those ugly ...
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1answer
203 views

Linux Server: Too Many Open Files

I have an application running on Linux Server. But server suddenly stopped responding but when I ping from other machine to server it allowed to ping but was not allowing to login into the server. Not ...
2
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1answer
243 views

Default value of /proc/sys/fs/file-max

I am aware that /proc/sys/fs/file-max defines the maximum number of open file descriptors and can be set on runtime or during boot. However: what is its default value? Checking 10 servers in my ...
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2answers
936 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 ...
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4answers
3k views

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?
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0answers
375 views

Debian Jessie 8 idle on fresh install always a little too high, shows rcu_sched ksoftirqd kworker causing cpu load

Are the days of 0.0 cpu load on an "idle" server gone? I have another server (centos/redhat) 32GB ram that actually runs apache/MySQL/Lamp, mono, redmine/ruby, and other junk and its still shows me 0....
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0answers
2k views

sysctl: cannot stat /proc/sys/kernel/exec-shield: No such file or directory

Enable ExecShield protection - ExecShield is security Linux kernel patch to avoid worms and other problems. After Adding to /etc/sysctl kernel.exec-shield = 1 Getting this error sysctl: cannot ...
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0answers
267 views

'BUG: unable to handle kernel NULL pointer dereference at' on Google Compute Engine

On a semi-regular basis, I've seen GCE instances freezing with the following error message (from the serial console): g[1375589.784755] BUG: unable to handle kernel NULL pointer dereference at ...
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2answers
455 views

Removing old kernels

My boot partition is full and I can't install new kernel versions. So I tried to remove some old versions like this: get active kernel uname -a Linux Ubuntu-1404-trusty-64-minimal 3.13.0-43-...
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1answer
503 views

NTP service stopped in centos7.1 linux machine

In our 3 linux boxes centos 7.1, we saw below messages in the logs and then ntpd service got stopped since the offset is more than 1000s. systemd: Time has been changed ntpd[2626]: 0.0.0.0 0617 07 ...
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1answer
221 views

How can I directly read data from a file in Linux without using the filesystem cache?

I have shared storage which is mounted (read-only) on SERVER1, and mounted (read-write) on SERVER2. SERVER2 is writing data to a file. SERVER1 is reading data from that file. The file poling is <...
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1answer
133 views

Write cache settings not working

I am using Debian 8.0 x64 with 72 GB of RAM with 4 drives configured as software RAID 0. Linux s1 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt11-1 (2015-05-24) x86_64 GNU/Linux I tried setting dirty ...
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1answer
169 views

Can't get rid of mystery kernel

Just noticed that despite having the kernel updated regularly, the version in use is quite outdated: $ uname -a Linux test 2.6.31-302-rs #7 SMP Thu Oct 29 22:57:03 UTC 2009 x86_64 x86_64 x86_64 GNU/...
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5answers
744 views

When does an upgrade update the Ubuntu kernel?

I'm a little confused by Ubuntu/Linux kernel updates. Coming from a Windows background I sort of expect all updates to be available via the update manager, in the case of Ubuntu apt/aptitude. Yet if ...
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3answers
3k views

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 ...
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244 views

Is transparent hugepage support on or off on these CoreOS machines?

I have a number of CoreOS servers that will be running some database workloads that highly recommend Transparent Hugepages be disabled. However: ~$ cat /sys/kernel/mm/transparent_hugepage/enabled ...
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0answers
445 views

strace not showing anything for thread at 100% 'system' CPU

I'm having difficulty with my Python app going to 100% 'system' CPU time across multiple threads for up to 10+ seconds. My (limited) understanding is that 'system' CPU time is time spent in the kernel....
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1answer
38 views

Linux kernel behavior when dealing with colliding subnets

I'm using openconnect to connect to my office VPN. They push some pretty crappy/aggressive routing rules allocating ALL of the private IP address spaces >.< After I connect to my office VPN, my ...
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2answers
2k views

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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0answers
46 views

How do processes stuck on uninterruptible sleep state get cleared upon reboot? [closed]

While processes stuck on an uninterruptible sleep state (i.e. TASK_UNINTERRUPTIBLE waiting I/O from a defunct device) cannot be killed by signals sent by root via the kill command, more often than not ...
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0answers
85 views

CPU usage of server extremely low and webpage load time high?

On a weekday at a time where very few use the optimized website on the server, the time for the page to completely load excluding 3rd party advertisements is about 0.6 seconds to 1 second. Now when a ...
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2answers
46 views

Can I use kernel modules on Amazon HVM/PV instances?

It's a perfectly stupid idea, I know. But I want to test one of my older projects, which includes kernel module (Netfilter extension) on a virtualized cloud-y instance. Will it be possible for me to ...
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1answer
589 views

Random machine hangs with NFSv4 on CentOS/RHEL 6.5

We have an in-house "compute farm" with about 100 CentOS (free re-distribution of RHEL) 5.7 and 6.5 x86_64 servers. (We are in the process of upgrading all the 5.7 boxes to 6.5.) All these machines ...
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2answers
226 views

AWS fails to resize an RHEL64 instance upon launch

I'm using AWS to boot up a RHEL64 instance but the disk is failing to resize to fit the volume I've selected. Here's the scenario: I Launch a running instance from the Marketplace AMI ami-517eec6b (...
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0answers
286 views

linux system slab cache proc_node_cache usage too large

My system run java program only, at some time the memory usage increase very quickly to a very hight percentage: memory usage of java : PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ ...
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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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0answers
2k views

rcu_sched self-detected stall on CPU on Debian server

My server is online since October 2014. Debian GNU/Linux 7.6 uname -r 3.10.23-xxxx-std-ipv6-64 It was working fine till last night. Last night it bacame unresponsible. I rebooted it from dadata ...
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0answers
82 views

Server crashes because of SMP

My web server is crashing every few days unless I disable SMP with "nosmp" parameter in Grub config. It is not an hardware failure because the problem persists after moving everything to a new machine....
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1answer
153 views

High %sys utilization in Nginx cache node

We set up Nginx with Lua (openresty bundle) to be a local caching node to our file sharing server, we seperated files to chunks "50MB each" (by this method) and store them in cache to increase its ...
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2answers
146 views

Can a Linux Kernel be shared between two operating systems?

Quite simple question. I am working on a project to have multiple operating systems (all linux) on the same machine using btrfs subvolumes so that the user can boot to the environment they are ...
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3answers
179 views

Networking in Linux KVM

I want to know if there is any way other than using linux bridges to interconnect interfaces from two virtual machines ? Since I am trying to run private spanning tree implementation in virtual ...
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0answers
525 views

“error: ‘PATH_MAX’ undeclared” when trying to make menuconfig

I have an OVH cloud VPS with Ubuntu 14.04 64 bit server. I needed linux headers which OVH does not provide by default, so I found some tutorials on how to compile the linux kernel from source using ...
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280 views

PCIe card memory regions ignored on linux kernel 3.13

On my system, with kernel 3.13.0-32-generic (Ubuntu Server 14.04.1 LTS) installed, one of the PCIe boards in the system (06:00.0) is having it's memory regions ignored: Interrupt: pin A routed to IRQ ...
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0answers
1k views

What are the pros and cons of large receive offload (LRO) vs generic receive offload (GRO)

I have been debugging a scenario where an ESX vmware host is communicating with a virtual linux machine via a virtual switch using TCP/IP. After a congestion event the vmware host is waiting for an ...
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 ...
2
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1answer
602 views

Running multiple ubuntu (or other distro) inside Docker containers on CoreOS, is this efficient?

From what I have read I believe CoreOS to be a bare-bones Linux distribution which essentially facilitates running Docker containers that contain any applications or services that you require. From ...
2
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0answers
161 views

What are the advantages NAPI before the IRQ Coalesce?

As known there are two approach to avoid some overheads of hardware interrupts in highload networks, when there are too many hardware interrupts, that switching to them takes too much time. It is very ...
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1answer
75 views

IPSEC stack low speed

I am working on Openswan IPSEC Implementation for Linux-kernel 2.6.35.9 My concern is when i do speed test using IPSEC kernel Images and without running IPSEC service i get speed of 40Mbps and 10Mbps....
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1answer
140 views

My server boots a kernel I don't recognize

On my dedicated server I'm running Debian 7.5 as an OS. I have installed OpenVZ for amd64 architecture to use the benefits of virtualization but after successfully installing it (following these ...
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1answer
1k views

Warning :mounting fs with errors, running e2fsck is recommended

Linux server will hang and when I restart it will take around 2 hours to come up and resume normal operations. I checked the system log files and found mounting fs with errors, running e2fsck is ...
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1answer
2k views

How to disable ipv6 support in Linux entirely?

We are currently setting up a some local network servers to run in a full ipv4 network. I can be sure, that this network don't have ipv6 support, and never will. Next to that, partially because of ...
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1answer
353 views

Windows multicast NLB with Linux clients

I'm (on behalf of a client, anyway) having some troubles with Linux accessing a multicast NLB cluster hosted on Hyper-V (waiting for more information on what version, but I don't think the version is ...