Re: 2.4.8preX VM problems

[Andrew Tridgell <tridge@valinux.com>]

Marcelo,

I've narrowed it down some more. If I apply the whole zone patch
except for this bit:

+		/* 
+		 * If we are doing zone-specific laundering, 
+		 * avoid touching pages from zones which do 
+		 * not have a free shortage.
+		 */
+		if (zone && !zone_free_shortage(page->zone)) {
+			list_del(page_lru);
+			list_add(page_lru, &inactive_dirty_list);
+			continue;
+		}
+

then the behaviour is much better:

[root@fraud trd]# ~/readfiles /dev/ddisk 
202 MB    202.125 MB/sec
394 MB    192.525 MB/sec
580 MB    185.487 MB/sec
755 MB    175.319 MB/sec
804 MB    41.3387 MB/sec
986 MB    182.5 MB/sec
1115 MB    114.862 MB/sec
1297 MB    182.276 MB/sec
1426 MB    128.983 MB/sec
1603 MB    164.939 MB/sec
1686 MB    82.9556 MB/sec
1866 MB    179.861 MB/sec
1930 MB    63.959 MB/sec

Even given that, the performance isn't exactly stunning. The
"dummy_disk" driver doesn't even do a memset or memcpy so it should
really run at the full memory bandwidth of the machine. We are only
getting a fraction of that (it is a dual PIII/800 server). If I get
time I'll try some profiling.

I also notice that the system peaks at a maximum of just under 750M in
the buffer cache. The system has 1.2G of completely unused memory
which I really expected to be consumed by something that is just
reading from a never-ending block device.

For example:

CPU0 states:  0.0% user, 67.1% system,  0.0% nice, 32.3% idle
CPU1 states:  0.0% user, 65.3% system,  0.0% nice, 34.1% idle
Mem:  2059660K av,  842712K used, 1216948K free,       0K shrd,  740816K buff
Swap: 1052216K av,       0K used, 1052216K free                    9496K cached

  PID USER     PRI  NI  SIZE  RSS SHARE LC STAT %CPU %MEM   TIME COMMAND
  615 root      14   0   452  452   328  1 R    99.9  0.0   3:52 readfiles
    5 root       9   0     0    0     0  1 SW   31.3  0.0   1:03 kswapd
    6 root       9   0     0    0     0  0 SW    0.5  0.0   0:04 kreclaimd

I know this is a *long* way from a real world benchmark, but I think
it is perhaps indicative of our buffer cache system getting a bit too
complex again :)

Cheers, Tridge