I wanted to be really able to explain to a fair amount of detail how does the program :command:`ls` actually work right from the moment you type the command name and hit ENTER. What goes on in user space and and in kernel space? This is my attempt and what I have learned so far on Linux (Fedora 19, 3.x kernel).
How does the shell find the location of 'ls' ?
| # Basically the nginx configuration I use at konklone.com. | |
| # I check it using https://www.ssllabs.com/ssltest/analyze.html?d=konklone.com | |
| # | |
| # To provide feedback, please tweet at @konklone or email eric@konklone.com. | |
| # Comments on gists don't notify the author. | |
| # | |
| # Thanks to WubTheCaptain (https://wubthecaptain.eu) for his help and ciphersuites. | |
| # Thanks to Ilya Grigorik (https://www.igvita.com) for constant inspiration. | |
| server { |
| import os | |
| def head(filename, count=1): | |
| """ | |
| This one is fairly trivial to implement but it is here for completeness. | |
| """ | |
| with open(filename, 'r') as f: | |
| lines = [f.readline() for line in xrange(1, count+1)] | |
| return filter(len, lines) |
L1 cache reference ......................... 0.5 ns
Branch mispredict ............................ 5 ns
L2 cache reference ........................... 7 ns
Mutex lock/unlock ........................... 25 ns
Main memory reference ...................... 100 ns
Compress 1K bytes with Zippy ............. 3,000 ns = 3 µs
Send 2K bytes over 1 Gbps network ....... 20,000 ns = 20 µs
SSD random read ........................ 150,000 ns = 150 µs
Read 1 MB sequentially from memory ..... 250,000 ns = 250 µs
| Latency Comparison Numbers (~2012) | |
| ---------------------------------- | |
| L1 cache reference 0.5 ns | |
| Branch mispredict 5 ns | |
| L2 cache reference 7 ns 14x L1 cache | |
| Mutex lock/unlock 25 ns | |
| Main memory reference 100 ns 20x L2 cache, 200x L1 cache | |
| Compress 1K bytes with Zippy 3,000 ns 3 us | |
| Send 1K bytes over 1 Gbps network 10,000 ns 10 us | |
| Read 4K randomly from SSD* 150,000 ns 150 us ~1GB/sec SSD |
This gist assumes:
| # Boyer Moore String Search implementation in Python | |
| # Ameer Ayoub <ameer.ayoub@gmail.com> | |
| # Generate the Bad Character Skip List | |
| def generateBadCharShift(term): | |
| skipList = {} | |
| for i in range(0, len(term)-1): | |
| skipList[term[i]] = len(term)-i-1 | |
| return skipList |