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Power efficiency as an analogue to memory management
Sara Alspaugh and Arka Bhattacharya
Conflicting goals?
• To save energy one needs to shut down servers
• Shutting down servers incurs huge latency penalty on active-set misses.
Take Lessons from Memory Management .
Memory Management
CACHE(<1 ns)
MEMORY DRAM
(~10 ns)
ACTIVE DISK(~10 ms)
P miss,cache
P miss,memory
Tavg= Tcache + Pmiss,cache .Tmemory + Pmiss,cache.Pmiss,memory .Tdisk
Penalty : 100 Penalty : 1000
REQ
UES
TS
Disk Management
CACHE(<1 ns)
MEMORY DRAM
(~100 ns)
ACTIVE DISK(~10 ms)
Tavg= Tcache + Pmiss,cache .Tmemory + Pmiss,cache.Pmiss,memory .Tdisk + Pmiss,cache.Pmiss,memory .Pmiss,active-disk.Tinactive-disk
IN-ACTIVE DISK
(~5 sec)
Pmiss,active-disk
P miss,cache
P miss,memory
Penalty : 100 Penalty : 1000
Penalty : 5000
REQ
UES
TS
For interactive jobs , average latency needed < 100 ms
Permissible active-disk misses
• Tavg ≈ Tactive-disk + Pmiss,active-disk.Tnon-active-disk
Permissible active-disk misses
• (100ms) ≈ (10ms) + Pmiss,active-disk.(5 sec)
Tactive-disk ≈ (10ms)Tnon-active-disk ≈ (5 sec)
For interactive jobs : Tavg < (100 ms)
Pmiss,active-disk < 2%
Permissible active-disk misses
• Tavg ≈ Tactive-disk + Pmiss,active-disk.Tnon-active-disk
TO CREATE ENERGY-PROPORTIONAL STORAGE , WE THUS HAVE TO MAINTAIN A NON-ACTIVE STORE AND REDUCE Pmiss,active
Techniques
1. Put disks to sleep after a loiter time , and wake-up on demand
2. Caching 3. Prefetching disk blocks4. Dynamic object creation/Intelligent layout
• Tavg ≈ Tactive-disk + Pmiss,active-disk.Tnon-active-disk
Wikipedia 1-day trace
• Replicated thrice• Partitioned into 5 disks
MSR-Cambridge Web/SQL server 7 day trace
• Not replicated• 4 volumes• 10 disks each
1 . Putting disks to sleep after Loiter
ACTIVE DISKACTIVE DISK ACTIVE DISK ACTIVE DISK
Request Stream
NON- ACTIVE
DISK
LOITER TIME ?
What loiter time to set ?
Energy Improvement
Latency Increase
Low Loiter TImes
High Loiter TImes Desired
100ms
Active-set misses vs aggressiveness of loiter
Putting it together
Active-disk misses
Do we really care about average latency ?
95th percentileLoiter Time = 100 sec
MSR
2 . Caching
File access frequency for top 5000 accessed files
5 205 405 605 805 100512051405160518052005220524052605280530053205340536053805400542054405460548050
1000
2000
3000
4000
5000
File Access frequency (Wikipedia)
Access frequency
5 255 505 755 1005 1255 1505 1755 2005 2255 2505 2755 3005 3255 3505 3755 4005 4255 4505 47550
1000
2000
3000
4000
5000
File access frequency - MSR Cambridge Web/SQL Server
Access Frequency
Options
1. Caching on an SSD2. Keep active set on and keep some spinning
reserve
CACHE(<1 ns)
MEMORY DRAM
(~10 ns)
ACTIVE DISK(~10 ms)
SSD cache(~50 μs)
REQ
UES
TS
What to cache on ?
DRAM SSD Hard Disk
Latency 5-10ns 60 μs 10ms
Cost / GB $100 $1-$2 5-10 ¢
Bandwidth 1500 MB/s 300-500 MB/s 50-100MB/s
Energy negligible 1-3W 10W
Increase in Average Idle Time with Increase in Cache Size
0 10000 50000 10000000
1
2
3
4
5
6
7
Wikipedia
CACHE SIZE
LARG
E ID
LE T
IME
(sec
)
0 100000 500000 10000000
50
100
150
200
250
300
350
400
450
MSR-Cambridge
CACHE SIZE
LARG
E ID
LE T
IME
(sec
)
How aggressive are you ? Different loiter times – (1) MSR
How aggressive are you ? Different loiter times – (2) Wikipedia
CACHE(<1 ns)
MEMORY DRAM
(~100 ns)
FIRST REPLICA(~10 ms)
IN-ACTIVE DISK
(~5 sec)
SPINNING RESERVE
Keeping an active set and spinning reserveRE
QU
ESTS
Active Set , Spinning Reserve , Normal
Wikipedia
3.Prefetching
Spatial correlation
Average : 0.3 %
TIM
E W
IND
OW
BLOCK NUMBER
Graph of different prefetching constants - Microsoft
MSR
Latency Analysis
MSR
4 . Intelligent layout of Objects
ACTIVE DISKACTIVE DISK ACTIVE DISK ACTIVE DISK
Request Stream
ACTIVE DISKACTIVE DISK ACTIVE DISK ACTIVE DISK
Request Stream
NON- ACTIVE
DISK
NON- ACTIVE
DISK
Wikipedia
Summary
• Not much different from memory management
• Very simple techniques which could be optimized on the trace.
• Large energy savings to be had if we are willing to pay slight latency penalties.
