Questions about server hardware and software performance, or network performance.

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112
votes
4answers
153k views

FreeBSD performance tuning: Sysctl parameter, loader.conf, kernel

I wanted to share knowledge of tuning FreeBSD via sysctl.conf / loader.conf / KENCONF / etc. It was initially based on Igor Sysoev's (author of nginx) presentation about FreeBSD tuning up to 100,000-...
82
votes
8answers
12k views

How can a single disk in a hardware SATA RAID-10 array bring the entire array to a screeching halt?

Prelude: I'm a code-monkey that's increasingly taken on SysAdmin duties for my small company. My code is our product, and increasingly we provide the same app as SaaS. About 18 months ago I moved ...
81
votes
13answers
25k views

Are networks now faster than disks?

This is a software design question I used to work on the following rule for speed cache memory > memory > disk > network With each step being 5-10 times the previous step (e.g. cache ...
80
votes
4answers
15k views

Why is TCP accept() performance so bad under Xen?

The rate at which my server can accept() new incoming TCP connections is really bad under Xen. The same test on bare metal hardware shows 3-5x speed ups. How come this is so bad under Xen? Can you ...
66
votes
8answers
144k views

What limits the maximum number of connections on a Linux server?

What kernel parameter or other settings control the maximum number of TCP sockets that can be open on a Linux server? What are the tradeoffs of allowing more connections? I noticed while load testing ...
63
votes
23answers
103k views

How to copy a large number of files quickly between two servers

I need to transfer a huge amount of mp3s between two serves (Ubuntu). By huge I mean about a million files which are on average 300K. I tried with scp but it would have taken about a week. (about 500 ...
54
votes
10answers
14k views

How to [politely?] tell software vendor they don't know what they're talking about

Not a technical question, but a valid one nonetheless. Scenario: HP ProLiant DL380 Gen 8 with 2 x 8-core Xeon E5-2667 CPUs and 256GB RAM running ESXi 5.5. Eight VMs for a given vendor's system. ...
47
votes
8answers
22k views

How much of a performance hit for https vs http for apache?

Roughly how much of a performance hit will https take compared to http for the same page? Suppose I can handle 1000 requests/s for abc.php, how much will it decrease by when accessed through https? I ...
38
votes
7answers
55k views

How can you know what is w3wp.exe doing? (or how to diagnose a performance problem)

I'm having a performance problem in a site we've made, and I'm not exactly sure how to start diagnosing it. The short description is: We have a very small site (http://hearablog.com) with very little ...
38
votes
5answers
33k views

Which PHP (opcode) cache one should use and why?

I keep hearing about some PHP (opcode) caches like - APC, XCache, Memcache, eAccelerator, etc. But I couldn't ever figure out how to go about choosing one. Apart from performance benefit, which a ...
37
votes
12answers
38k views

Is virtual machine slower than the underlying physical machine?

This question is quite general, but most specifically I'm interested in knowing if virtual machine running Ubuntu Enterprise Cloud will be any slower than the same physical machine without any ...
36
votes
6answers
29k views

Software vs hardware RAID performance and cache usage

I've been reading a lot on RAID controllers/setups and one thing that comes up a lot is how hardware controllers without cache offer the same performance as software RAID. Is this really the case? I ...
34
votes
7answers
60k views

Xen vs KVM in performance

What is faster on same hardware - Xen or KVM? I'm trying to pick-up a virtualization technology to work, which gives the best performance. There are some benchmarks here that I found on the subject: ...
34
votes
5answers
1k views

Pay no attention to that SAN behind the curtain

Once upon a time, I built my own SQL servers, and had control over drive configuration, RAID levels, etc. The traditional advice of separation of data, logs, tempdb, backups, (depending on budget!) ...
32
votes
2answers
4k views

Why is my bare-metal 16x 2.93GHz cores computer performing poorer than a VPS with 4x 2.5GHz cores?

I have a written a piece of multi-threaded software that does a bunch of simulations a day. This is a very CPU-intensive task, and I have been running this program on cloud services, usually on ...
29
votes
5answers
4k views

Why does it take tens of seconds to get a shell prompt?

It's a sort of regular occurrence that, after SSHing to a server (or even opening a terminal on my Mac), the login banner prints immediately but it takes ~10 seconds to a minute for the shell prompt ...
29
votes
6answers
86k views

High load average, low CPU usage - why?

We're seeing huge performance problems on a web application and we're trying to find the bottleneck. I am not a sysadmin so there is some stuff I don't quite get. Some basic investigation shows the ...
29
votes
11answers
23k views

How to make `rm` faster on ext3/linux?

I have ext3 filesystem mounted with default options. On it I have some ~ 100GB files. Removal of any of such files takes long time (8 minutes) and causes a lot of io traffic, which increases load on ...
28
votes
9answers
10k views

To improve SQL performance, why not just put lots of RAM rather than having faster hard disks?

People keep telling me that in order to improve an SQL server's performance, buy the fastest hard disks possible with RAID 5, etc. So I was thinking, instead of spending all the money for RAID&...
28
votes
3answers
48k views

Why is my rsync so slow?

My Laptop and my workstation are both connected to a Gigabit Switch. Both are running Linux. But when I copy files with rsync, it performs badly. I get about 22 MB/s. Shouldn't I theoretically get ...
28
votes
7answers
15k views

What are the performance differences between Raid 0,1,5,6,10

I have heard/read different performance stories on the various raid flavors. I am curious what the agreed upon best answer is.
28
votes
1answer
24k views

Is there still a use for irqbalance on modern hardware?

This question has been asked before, but I believe that the world has changed enough for it to be asked again. Does irqbalance have any use on today’s systems where we have NUMA-capable CPUs with ...
26
votes
3answers
9k views

PHP: What are the advantages of FastCGI over mod_php?

It was recently suggested to me that I use FastCGI with PHP. Now I went to the FastCGI page and read it but I don't really understand what the advantages are.
25
votes
7answers
11k views

Is it OK to run perfmon on production servers? And why?

Or should perfmon be limited to a Dev/QA server with load tests that simulate production activity? I'd like to run perfmon for two days (like Sql Server master Brent Ozar suggests) to get an overall ...
24
votes
11answers
32k views

What can cause slow ssh session? [closed]

I ssh on remote host but terminal performance is poor. Symbols I am typing are not shown immediately, but with some delay. Sometimes two symbols are shown at one time after delay.
24
votes
5answers
21k views

Process runs slower as a scheduled task than it does interactively

I have a scheduled task which is very CPU- and IO-intensive, and takes about four hours to run (building source code, if you're curious). The task is a Powershell script which spawns various sub-...
23
votes
5answers
27k views

Can a pool of memcache daemons be used to share sessions more efficiently?

We are moving from a 1 webserver setup to a two webserver setup and I need to start sharing PHP sessions between the two load balanced machines. We already have memcached installed (and started) and ...
23
votes
4answers
15k views

Why VM snapshots are affecting performance?

I read in one of the VMware KB articles that snapshots will directly affect VM performance. But my team keeps asking me how snapshots can affect performance. I would like to give them solid reason ...
23
votes
5answers
46k views

How many Context Switches is “normal” (as a function of CPU cores (or other))?

Do any of you have a rule of thumb as to how many context switches (per processor core) is Normal on a Linux server? My college here brought it up, and he's seeing 16K on a 8-core x86_64 machine. ...
22
votes
17answers
41k views

What network file sharing protocol has the best performance and reliability? [closed]

We have a setup with a few web servers being load-balanced. We want to have some sort of network shared storage that all of the web servers can access. It will be used as a place to store files ...
22
votes
3answers
16k views

OpenVPN performance: how many concurrent clients are possible?

I am evaluating a system for a client where many OpenVPN clients connect to a OpenVPN server. "Many" means 50000 - 1000000. Why do I do that? The clients are distributed embedded systems, each ...
22
votes
3answers
52k views

Optimal values for ServerLimit, MaxClients, MaxRequestsPerChild directives

I'm running a traffic intense site plenty of dynamic content, mostly user-generated. The server is a dedicated one and has a total of 4 Intel(R) Xeon(R) CPU X3210 @ 2.13GHz proccesors. I need to know ...
21
votes
4answers
24k views

How to pick between RabbitMQ and ZeroMQ or something else?

I'm looking for a queuing system which is fast and reliable and potentially can be spread across machines. Platform is Linux. Open source is preferable. Both RabbitMQ and ZeroMQ look good but I ...
20
votes
4answers
2k views

Why would a heavily disk intensive application run faster on a SAN than on a Physical Disk?

Why would a heavily disk intensive application run faster on a SAN than on a Physical Disk? I would have expected the Physical disk to be slightly faster but in fact the process ran 100 times faster ...
20
votes
8answers
31k views

What impact does full hard drive encryption have on performance?

We have HP notebooks here at work, and it is policy to have HP's hard drive encryption turned on to protect client databases and IP in the case of loss/theft. I was wondering if there was any ...
20
votes
3answers
19k views

limit linux background flush (dirty pages)

Background flushing in linux happens when either too much written data is pending (adjustable via /proc/sys/vm/dirty_background_ratio) or a timeout for pending writes is reached (/proc/sys/vm/...
20
votes
1answer
660 views

Why higher await time for DM multipath device than underlying device?

We have a CentOS 6.4 based server attached to Hitachi HNAS 3080 storage and observed the kernel remount the filesystem in read-only mode: May 16 07:31:03 GNS3-SRV-CMP-001 kernel: [1259725.675814] ...
19
votes
5answers
2k views

Extremely fast booting server

I'm changing the way that our DHCP/DNS stuff works at work. Currently we've got 3 DNS servers, and a DHCP box. All of them are VMs. There's a circular dependency where stuff booting requires NFS, ...
19
votes
2answers
5k views

How are CPU time and CPU usage the same?

In the Wikipedia page for CPU time, it says The CPU time is measured in clock ticks or seconds. Often, it is useful to measure CPU time as a percentage of the CPU's capacity, which is called ...
18
votes
8answers
10k views

Doing an rm -rf on a massive directory tree takes hours

We're using rsnapshot for backups. It keeps lots of snapshots of the backuped up file, but it does delete old ones. This is good. However it's taking about 7 hours to do a rm -rf on a massive ...
18
votes
11answers
23k views

How to monitor and log the memory/cpu usage of processes over time? [closed]

I am looking for a way to diagnose issues, such as swap death, where a balooning memory process fills up swap and kills the whole machine (such as apache). I'm already using cacti and I can set up ...
18
votes
11answers
11k views

Are there any good reasons for disabling hardware-assisted virtualization?

We've had a number of servers from Dell recently, all of which have had hardware-assisted virtualization disabled in the BIOS. As far as I know hardware-assisted virtualization is a good thing - so ...
18
votes
8answers
15k views

What is the best filesystem for insert performance on PostgreSQL?

I'm curious if anyone out there has done any experimentation or comparisons between file systems and database performance. On Linux, I'm wondering what is the optimal file system for a postgres ...
18
votes
7answers
30k views

How can I disable encryption on openssh?

I'm having performance problems using openshh(server) and putty(client) combination to use a remote webproxy. I'd like to disable encryption and test the results to see if it makes a difference. How ...
18
votes
1answer
27k views

Determining which process is causing heavy disk I/O?

I've seen this question: How to identify heavy write to disk? And I've used dstat and atop before...but they don't seem to pinpoint what process is causing disk I/O. For example, from dstat: dstat ...
18
votes
3answers
13k views

How much overhead does x86/x64 virtualization have?

How much overhead does x86/x64 virtualization (I'll probably be using VirtualBox, possbly VMWare, definitely not paravirtualization) have for each of the following operations a Win64 host and Linux64 ...
18
votes
8answers
16k views

Analyzing Linux NFS server performance

I'd like to do some analysis of our NFS server to help track down potential bottlenecks in our applications. The server is running SUSE Enterprise Linux 10. The kind of things I'm looking to know are:...
16
votes
2answers
1k views

Can a virtualized machine have the CPU and RAM resources of multiple underlying physical machines?

We are studying implementing some virtualized servers here, but we don't know what will be better suitable for us. Some folks are saying better have two huge servers, and others are saying have like a ...
16
votes
4answers
29k views

How to identify I/O bottlenecks on a Linux server?

How do you determine that the performance of your Linux server is I/O bound and, perhaps more importantly, what process or processes are casuing the problem?
16
votes
9answers
17k views

How do I profile MySQL?

What tools exist to profile MySQL, like how MSSQL 2000+ does with the SQL Profiler? I'd want to trace things like SQL statements executed, execution times, execution plan, etc.