Monday, April 30, 2012

How to find the upstream website of a package in Fedora or any other rpm based linux distribution?

What is upstream in opensource?
Package maintainers in linux distributions like Fedora would build and submit rpm packages of any opensource software which they think would be useful to be included in Fedora. The people who originally created the software is called upstream.

How to see upstream website url?
The website url of the upstream software maintainer is included in the rpm spec file. The url can be seen in the output of 'rpm -qi command'. For example, to see the upstream url of vpnc rpm package.

$ rpm -qi vpnc|grep URL
URL         :

Friday, September 10, 2010

Why dmesg is showing write through when the disk connected to Adaptec AAC-RAID is set to write back as per Adaptec Storage Manager?

dmesg command is showing following output.

Vendor: Adaptec Model: Data Rev: V1.0
Type: Direct-Access ANSI SCSI revision: 02
sdb : very big device. try to use READ CAPACITY(16).
SCSI device sdb: 9762222080 512-byte hdwr sectors (4998258 MB)
sdb: Write Protect is off
sdb: Mode Sense: 06 00 10 00
SCSI device sdb: drive cache: write through w/ FUA
sdb : very big device. try to use READ CAPACITY(16).
SCSI device sdb: 9762222080 512-byte hdwr sectors (4998258 MB)
sdb: Write Protect is off
sdb: Mode Sense: 06 00 10 00
SCSI device sdb: drive cache: write through w/ FUA
sdb: sdb1
sd 0:0:1:0: Attached scsi removable disk sdb

This is expected behavior.

What the aacraid driver reports is independent of the firmware settings configured by the storage manager. Instead, the write back configuration the aacraid driver reports is controlled the variable aac_cache (which can be set by a module parameter named cache).

From drivers/scsi/aacraid/aachba.c:

static int aac_cache = 2; /* WCE=0 to avoid performance problems */


module_param_named(cache, aac_cache, int, S_IRUGO|S_IWUSR);
MODULE_PARM_DESC(cache, "Disable Queue Flush commands:\n"
"\tbit 0 - Disable FUA in WRITE SCSI commands\n"
"\tbit 1 - Disable SYNCHRONIZE_CACHE SCSI command\n"
"\tbit 2 - Disable only if Battery is protecting Cache");

With the default value of 2, aacraid always reports disks as being in write through mode.

Wednesday, August 4, 2010

Windows telnet client displays garbage characters when connected to Red Hat Enterprise Linux

When user connects to Red Hat Enterprise Linux Server 5 using Windows telnet client, and compiles C programs, junk or garbage characters are displayed.

[test@test ~]$ gcc test.c   
test1.c: In function ΓÇÿmainΓÇÖ:

When one connects via telnet from a Windows system to Red Hat Enterprise Linux 5 system, the telnet client uses the character encoding from Windows. This results in the strange characters. Windows does not use utf-8 character encoding like RHEL.

Using putty on Windows as the telnet client solves the issue. In putty, go to Translation -- > change 'ISO-8859-1:1998 (Latin-1, West Europe)' to UTF-8 and then connect .

Wednesday, July 28, 2010

How to stop the flushing of mmaped pages to disk in RHEL5.2 and later?

If your application is mmapping too much memory, resulting in lots of pdflush write activity and thereby by affecting system performance, vm.flush_mmap_pages kernel parameter could be useful.

/proc/sys/vm/flush_mmap_pages is introduced in RHEL5.2 kernel.

Setting /proc/sys/vm/flush_mmap_pages to 0 will stop the flushing of dirty memory-mapped file pages to disk as long as the memory map is active.

All dirty file pages will be asynchronously flushed to disk only as soon as the memory map is deactivated.

Added a new kernel parameter: /proc/sys/vm/flush_mmap_pages. This parameter specifies whether or not memory-mapped file pages should be flushed to disk by kupdate while the memory map is active. Valid values for this parameter are 1 (enable memory mapping by kupdate) and 0 (disable memory mapping by kupdate). The default value for this parameter is 1.

To configure this parameter, use echo [1 or 0] /proc/sys/vm/flush_mmap_pages. Setting this parameter to 0 does the following:

kupdate will not flush dirty memory-mapped file pages as long as the memory map is active.

All dirty file pages will be asynchronously flushed to disk only as soon as the memory map is deactivated.

If you set /proc/sys/vm/flush_mmap_pages to 0, it is advisable that you use another application to manually sync memory-mapped pages to disk.

Friday, June 11, 2010

byobu - new funky wrapper for screen command

screen command was always been a very helpful tool for system admins.

byobu is a wrapper script over screen.

Best part is that it looks good.

You can also configure status notifications which will be displayed in the bottom screen. This is useful in monitoring the system while working in the terminal.

In Fedora, you can install byobu using following command.
# yum install byobu

Some useful shortcuts from man byobu

F2 - Create a new window

F3 - Move to previous window

F4 - Move to next window

F5 - Reload profile

F6 - Detach from this session

F7 - Enter copy/scrollback mode

F8 - Re-title a window

F9 - Configuration Menu

F12 - Lock this terminal
Run 'byobu' command to start the script.

Thursday, May 27, 2010

How do you find out which cpu a process is running on?

The output of the ps command can be changed to a user defined format by using the -o option.

The following command can be used to display the CPU and the processes assigned.

# ps -eo pid,args,psr


pid - The process ID of the process.

args - Command with all its arguments as a string.

psr - processor currently assigned to the process.

Go to the man pages for more information on the ps command.

What is the logic behind killing processes during an Out of Memory situation?

As per kernel source code, following is OOM-killer logic,

A function called badness() is defined to calculate points for each processes.

* Points will be added to following processes.

Processes with high memory size.
Niced processes.

* Points will be reduced from following processes.

Processes which were running for long time.
Processes which were started by superusers.
Process with direct hardware access.

The process with the highest number of point, will be killed, unless it is already in the midst of freeing up memory on its own.

Then the system will wait for sometime to see if enough memory is freed. If enough memory is not freed after killing one process, the above steps will continue.

As per select_bad_process function, if a processes is having 0 or less points it could not be killed. The oom kills will be continued until there is no candidate processes left to kill. If the system is not able to find a candidate process to kill, it panics.

static unsigned long badness(struct task_struct *p, unsigned long uptime)
unsigned long points, cpu_time, run_time, s;

if (!p->mm)
return 0;

if (p->flags & PF_MEMDIE)
return 0;
* The memory size of the process is the basis for the badness.
points = p->mm->total_vm;

* CPU time is in tens of seconds and run time is in thousands
* of seconds. There is no particular reason for this other than
* that it turned out to work very well in practice.
cpu_time = (p->utime + p->stime) >> (SHIFT_HZ + 3);

if (uptime >= p->start_time.tv_sec)
run_time = (uptime - p->start_time.tv_sec) >> 10;
run_time = 0;

s = int_sqrt(cpu_time);
if (s)
points /= s;
s = int_sqrt(int_sqrt(run_time));
if (s)
points /= s;

* Niced processes are most likely less important, so double
* their badness points.
if (task_nice(p) > 0)
points *= 2;

* Superuser processes are usually more important, so we make it
* less likely that we kill those.
if (cap_t(p->cap_effective) & CAP_TO_MASK(CAP_SYS_ADMIN) ||
p->uid == 0 || p->euid == 0)
points /= 4;

* We don't want to kill a process with direct hardware access.
* Not only could that mess up the hardware, but usually users
* tend to only have this flag set on applications they think
* of as important.
if (cap_t(p->cap_effective) & CAP_TO_MASK(CAP_SYS_RAWIO))
points /= 4;
#ifdef DEBUG
printk(KERN_DEBUG "OOMkill: task %d (%s) got %d points\n",
p->pid, p->comm, points);
return points;