[uml-user] Disk writes causes UML to hang

[Evan J. <ej...@uw...>]

I was hoping that someone on this list might have some suggestions for 
what settings could be tweaked to prevent or to alleviate this problem:

When I perform some operation on the UML that does a lot of disk writes 
(for example, copying a large file), the UML will become unresponsive 
until the write has completed. Running "vmstat 2" will pause, and when 
it returns it shows a huge number of block writes. Pings to the UML 
will have huge latency. I'm using an ext3 UML disk that has noatime 
set. The UML is running 2.4.24-2um.

I'm guessing that the hang occurs because my UML is asking the host to 
"sync" a write to disk, and is hanging until that sync operation 
completes. Does that seem reasonable? Could using ext2 help? Tweaking 
the bdflush values? Any other ideas?

Thank you for any assistance,

Evan Jones


Example:

ej@ej:~$ time cp old-websites.tar.bz2 test.tar.bz2  [copy a 9MB file]

real    0m0.301s
user    0m0.000s
sys     0m0.269s
[ the operation itself completes VERY quickly, but immediately after 
the UML hangs ]

In another console:

ej@ej:~$ vmstat 2
procs -----------memory---------- ---swap-- -----io---- --system-- 
----cpu----
  r  b   swpd   free   buff  cache   si   so    bi    bo   in    cs us 
sy id wa
  0  0  23228   3036   3184  25396    1    0     2     1    8     5  0  
0 100  0
  0  0  23228   3036   3184  25396    0    0     0     0   56     8  0  
0 100  0
  0  0  23228   3036   3184  25396    0    0     0     0   57    11  0  
1 99  0
  0  0  23228   3036   3184  25396    0    0     0     0   54     5  0  
0 100  0
  0  0  23228   1668   3272  26464    0   98     0    98   56    12  0 
13 87  0
[ NO OUTPUT AS THE UML HANGS HERE FOR A MINUTE ]
  3  0  23228   1612   3276  26464    0    0     0  3919 1835     7  0 
99  1  0
  0  0  23228   1580   3304  26500    0    0    19    41   54    19 20  
6 74  0
  0  0  23228   1576   3304  26500    0    0     0     0   54     2  0  
0 100  0
  0  0  23228   1572   3304  26500    0    0     0     0   56     4  0  
0 100  0
  0  0  23228   1572   3304  26500    0    0     0     0   55     2  0  
0 100  0
  0  0  23228   1572   3304  26500    0    0     0     0   54     5  0  
0 100  0



 From another machine:

PING evanjones.ca: 56 data bytes
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=0. time=86. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=1. time=86. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=2. time=85. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=3. time=85. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=4. time=86. ms
[ HUGE HANG ]
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=5. time=68608. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=6. time=67609. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=7. time=66609. ms
64 bytes from evanjones.ca (66.160.135.56): icmp_seq=8. time=65609. ms
[ MANY MORE LINES OF DELAYED PING REPLIES ]

--
Evan Jones: http://www.eng.uwaterloo.ca/~ejones/
"Computers are useless. They can only give answers" - Pablo Picasso