+++ /dev/null
-Adding support for "compact" 32 bits events.
-
-Mathieu Desnoyers
-March 12, 2007
-
-Use a separate channel for compact events
-
-Mux those events into this channel and magically they are "compact". Isn't it
-beautiful.
-
-event header
-
-### COMPACT EVENTS
-
-32 bits header
-Aligned on 32 bits
- 5 bits event ID
- 32 events
- 27 bits TSC (cut MSB)
- wraps 32 times per second at 4GHz
- each wraps spaced from 0.03125s
- 100HZ clock : tick each 0.01s
- detect wrap at least each 3 jiffies (dangerous, may miss)
- granularity : 2^0 = 1 cycle : 0.25ns @4GHz
-payload size known by facility
-
-32 bits header
-Aligned on 32 bits
- 5 bits event ID
- 32 events
- 27 bits TSC (cut LSB)
- wraps each second at 4GHz
- 100HZ clock : tick each 0.01s
- granularity : 2^5 = 32 cycles : 8ns @4GHz
-payload size known by facility
-
-32 bits header
-Aligned on 32 bits
- 6 bits event ID
- 64 events
- 26 bits TSC (cut LSB)
- wraps each 0.5 second at 4GHz
- 100HZ clock : tick each 0.01s
- granularity : 2^6 = 64 cycles : 16ns @4GHz
-payload size known by facility
-
-32 bits header
-Aligned on 32 bits
- 7 bits event ID
- 128 events
- 25 bits TSC (cut LSB)
- wraps each 0.5 second at 4GHz
- 100HZ clock : tick each 0.01s
- granularity : 2^7 = 128 cycles : 32ns @4GHz
-payload size known by facility
-
-
-
-### NORMAL EVENTS
-
-64 bits header
-Aligned on 64 bits
- 32 bits TSC
- wraps each second at 4GHz
- 100HZ clock : tick each 0.01s
- 16 bits event id, (major 8 minor 8)
- 65536 events
- 16 bits event size (extra)
-
-96 bits header (full 64 bits TSC, useful when no heartbeat available)
-Aligned on 64 bits
- 64 bits TSC
- wraps each 146.14 years at 4GHz
- 16 bits event id, (major 8 minor 8)
- 65536 events
- 16 bits event size (extra)
-
-
-## Discussion of compact events
-
-Must put the event ID fields first in the large (64, 96-128 bits) event headers
-What is the minimum granularity required ? (so we know how much LSB to cut)
- - How much can synchonized CPU TSCs drift apart one from another ?
- PLL
- http://en.wikipedia.org/wiki/Phase-locked_loop
- static phase offset -> tracking jitter
- 25 MHz oscillator on motherboard for CPU
- jitter : expressed in ±picoseconds (should therefore be lower than 0.25ns)
- http://www.eetasia.com/ART_8800082274_480600_683c4e6b200103.HTM
- NEED MORE INFO.
- - What is the cacheline synchronization latency between the CPUs ?
- Worse case : Intel Core 2, Intel Xeon 5100, Intel core solo, intel core duo
- Unified L2 cache. http://www.intel.com/design/processor/manuals/253668.pdf
- Intel Core 2, Intel Xeon 5100
- http://www.intel.com/design/processor/manuals/253665.pdf
- Up to 10.7 GB/s FSB
- http://www.xbitlabs.com/articles/mobile/display/core2duo_2.html
- Intel Core Duo Intel Core 2 Duo
- L2 cache latency 14 cycles 14 cycles
- (round-trip : 28 cycles) 7ns @4GHz
- sparc64 : between threads : shares L1 cache.
- suspected to be ~2 cycles total (1+1) (to check)
- - How close (cycle-wise) can be two consecutive recorded events in the same
- buffer ? (~200ns, time for logging an event) (~800 cycles @4GHz)
- - Tracing code itself : if it's at a subbuffer boundary, more check to do.
- Must see the maximum duration of a non interrupted probe.
- Worse case (had NMIs enabled) : 6997 cycles. 1749 ns @4GHz.
- TODO : test with NMIs disabled and HT disabled.
- Ordering can be changed if an interrupt comes between the memory operation
- and the tracer call. Therefore, we cannot rely on more precision than the
- expected interrupt handler duration. (guess : ~10000cycles, 2500ns@4GHz)
- - If there is a faster interconnect between the CPUs, it can be a problem, but
- seems to only be proprietary interconnects, not used in general.
- - IPI are expected to take much more than 28 cycles.
-What is the minimum wrap-around interval ? (must be safe for timer interrupt
-miss and multiple timer HZ (configurable) and CPU MHZ frequencies)
-
-Granularity : 800ns (200 cycles@4GHz) : 2^9 = 512 (remove 9 LSB)
- Probe never takes 1 cycle.
- Number of LSB skipped : max(0, (long)find_first_bit(probe_duration_in_cycles)-1)
-
-Min wrap : 100HZ system, each 3 timer ticks : 0.03s (32-4 MSB for 4 GHZ : 0.26s)
- (heartbeat each 100HZ, to be safe)
- Number of MSB to skip :
- 32 - find_first_bit(( (expected_longest_interrupt_latency()[ms] +
- max_timer_interval[ms]) * cpu_khz[kcycles/s] )) - 1
- (the last -1 is to make sure we remove less or exact amount of bits, round
- near to 0, not round up).
-
-Heartbeat timer :
- Each timer interrupt
- Event : 32 bytes in size
- each timer tick : 100HZ
- 3.2kB/s
-
-9LSB + 4MSB = 13 bits total. 13 bits for event IDs : 8192 events.
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projects / lttv.git / blobdiff