time

1
Detours
On a time-sharing system, a process is
interrupted at any time.
time
Detour is time period not for a target process,
which contains

• kernel service
• scheduler,memory management,network subsystem,etc
• other process
• etc
• cache miss, TLB miss

2
Record each detour in precious
start1
start2
start3
start4
...
...
startN
time
duration1
duration2
duration3
duration3
durationN
record detours
detour recorder
output a recorded data
HEADERS TICKS_PER_SECOND ... start1
duration1 start2 duration2 start3 duration3 .....
3
Detour Recording Logic idea
Basic Idea meta code
rdtsc() is called frequently Some experimental
results, Running code at BG/L IO node 700MHz
PowerPC Linux 2.4.19 based kernel rdtsc() is
called every 0.1usec Minimum detour activety is
1.8 usec, which is timer update. If array is
2000 in size(1000 detours), it can record approx.
10 sec.
current rdtsc() while( array is not full )
prev current current rdtsc() if(
current prev gt precision )
array.push_back(prev) array.push_back(current
)
4
Graph - timeseries
60ms
10ms
5
Graph sorted by detour duration
60ms periodic event
10ms periodic event
6
Detours in fixed time quantum
time
Detours
Q
Total detour in Q
Sync Frequency 1/Q (HZ)
7
A node in parallel machine has detour. Detour
distribution is same as single machine.
node1
node2
node3
nodeN-1
nodeN
pick detour randomly from single node
experimental.
fixed time
record
changing sync. freq.
8
Simulation algorithm for parallel machine (from
a single node data)
node1
node2
node3
nodeN-1
nodeN
max(D)-min(D)
try 1
d1
d2
d3
dN-1
dN
max(D)-min(D)
try 1
d1
d2
d3
dN-1
dN
max(D)-min(D)
try M
d1
d2
d3
dN-1
dN
Input recorded data Nmachine size Mtry
count FSync Frequency
Output average and standard deviation of
max(D)-min(D)
9
What we know so far

• maximum detour(latency) is a matter of parallel
  perfomance
• minimize the max latency increase the parallel
  performance
• linux kernel may be good for parallel machine