Messages in this thread | | | Date | Mon, 31 Jul 2000 00:20:56 +0100 (BST) | From | Tigran Aivazian <> | Subject | amazing but true (2.4.0-test5) |
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Hi guys,
I remember a long time ago I found a rather interesting situation, whereby running a COBOL (microfocus) application while having a key pressed makes the processing go faster by about 100% or more. As it turned out it was due to the fact that some analogue of PC getch() was provided on top of select(2) (in order to check on each loop iteration if user pressed ESC key) and so if you don't press a key it will just keep timing out and so most of the work by application consists of timing out select, whilst if you do press a key, select(2) will return very quickly and application gets a lot more chance to do some work.
Now, why do I tell you this? Because this very day I discovered a very similar (yet completely different and for a completely different reason, currently unknown) effect.
I have an old Pentium 120MHz 32M RAM running 2.4.0-test5 and 100Mbit ATX-2500 card driven by 8139too driver. Now, when this card is used between fast machines (PII/375MHz talking to dual PIII/550MHz) everything is fine and I get 7M/second (Mbyte not Mbit!) ftp rates and 9Mbyte/sec bonnie over NFS which is close to theoretical limit (I wouldn't expect from Linux anything other than being perfect).
But when used on a slow P120MHz I only get 600Kbyte/second ftp transfer speeds. Moreover, I noticed that kswapd actually uses some CPU, as much as 20-30% thereof! And both the disk and network blinking was in bursts, on average resulting in about 600K/seconds.
Then, I accidentally left my finger on a key in a bash session running over rlogin to the slow box and, behold, the lights became solid (network and hard disk) and the ftp rate jumped up to 1500Kbyte/second. I experimented about 5 times on and off to make sure it is not a dream :)
It is reproducible - holding a key pressed makes my ftp sessions 2.5 times faster... It is too late now to tcpdump the situation or put some printks in 8139too driver to see if delays are due to network or is it some local effect (memory/io subsystem on slow machine) - maybe tomorrow.
Regards, Tigran
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