Le 08/10/10 22:02, Daniel Stenberg a écrit : > Well, on a modern PC that sends a small packet compressed, is the very > tiny microsecond or whatever the compression function call takes really > a factor at all? Probably. but compressing certainly is as costly as encryption. And can't be speeded up with hardware or threading tricks (like aes128-ctr which can be pre-computed by a thread on another cpu/core).
fyi: a...@darkforce:~$ dd if=/dev/urandom of=file count=1000000 1000000+0 enregistrements lus 1000000+0 enregistrements écrits 512000000 octets (512 MB) copiés, 73,3125 s, 7,0 MB/s a...@darkforce:~$ time dd if=file of=/dev/null [...] real 0m0.525s a...@darkforce:~$ time gzip -c file > /dev/null real 0m20.975s user 0m20.740s sys 0m0.130s on model name : AMD Phenom(tm) II X4 945 Processor I'm sorry I don't have statistics for aes encryption. > > I'm not saying that it does any good, I just question that it actually > is noticable. I guess if you send a large pre-compressed stream it will > be measureable at least. Here the throughput I got from my gzip (on a quite worst case, I ack) is around 200mbps (512000000 * 8 /20 /(1024*1024)). It's less than the speed I'd expect to have using SSH on high speed link, and it doesn't even count time spent on encryption and basic protocol handling. > >> My opinion is that compression is useful when transferring files of >> unknown type, or mainly text files. Some content with high entropy >> (compressed files, video, ...) are slow to compress and decompress ; >> If there is an API call do enable/disable it (libssh does), everybody >> should be happy. > > Right, but the question right now is mostly: what is the default? > > Out of curiosity, what does libssh default to? > It defaults to "none". If application designer thinks it's interesting to have compression by default, it's easy to enable. Also, it's a lot less of complicated code that is running in the lib and makes the debugging really easier when some bug report comes in (even if I don't remember having a bug report related to compression, except maybe the feature request for [email protected]). Aris _______________________________________________ libssh2-devel http://cool.haxx.se/cgi-bin/mailman/listinfo/libssh2-devel
