lttv/doc/developer/tsc-smallv1.txt

   1 Adding support for "compact" 32 bits events.
   2
   3 Mathieu Desnoyers
   4 March 9, 2007
   5
   6
   7 event header
   8
   9
  10 32 bits header
  11 Aligned on 32 bits
  12   1 bit to select event type
  13   4 bits event ID
  14     16 events (too few)
  15   27 bits TSC (cut MSB)
  16     wraps 32 times per second at 4GHz
  17     each wraps spaced from 0.03125s
  18     100HZ clock : tick each 0.01s
  19       detect wrap at least each 3 jiffies (dangerous, may miss)
  20     granularity : 2^0 = 1 cycle : 0.25ns @4GHz
  21 payload size known by facility
  22
  23 32 bits header
  24 Aligned on 32 bits
  25   1 bit to select event type
  26   4 bits event ID
  27     16 events (too few)
  28   27 bits TSC (cut LSB)
  29     wraps each second at 4GHz
  30     100HZ clock : tick each 0.01s
  31     granularity : 2^5 = 32 cycles : 8ns @4GHz
  32 payload size known by facility
  33
  34 32 bits header
  35 Aligned on 32 bits
  36   1 bit to select event type
  37   5 bits event ID
  38     32 events
  39   26 bits TSC (cut LSB)
  40     wraps each 0.5 second at 4GHz
  41     100HZ clock : tick each 0.01s
  42     granularity : 2^6 = 64 cycles : 16ns @4GHz
  43 payload size known by facility
  44
  45 32 bits header
  46 Aligned on 32 bits
  47   1 bit to select event type
  48   6 bits event ID
  49     64 events
  50   25 bits TSC (cut LSB)
  51     wraps each 0.5 second at 4GHz
  52     100HZ clock : tick each 0.01s
  53     granularity : 2^7 = 128 cycles : 32ns @4GHz
  54 payload size known by facility
  55
  56 64 bits header
  57 Aligned on 32 bits
  58   1 bit to select event type
  59   15 bits event id, (major 8 minor 8)
  60      32768 events
  61   16 bits event size (extra)
  62   32 bits TSC
  63     wraps each second at 4GHz
  64     100HZ clock : tick each 0.01s
  65
  66 96 or 128 bits header (full 64 bits TSC, useful when no heartbeat available
  67   size depends on internal alignment)
  68 Aligned on 32 bits
  69   1 bit to select event type
  70   15 bits event id, (major 8 minor 8)
  71      32768 events
  72   16 bits event size (extra)
  73 Align on 64 bits
  74   64 bits TSC
  75     wraps each 146.14 years at 4GHz
  76
  77
  78
  79
  80
  81 Must put the event ID fields first in the large (64, 96-128 bits) event headers
  82 Create a "compact" facility which reserves the facility IDs with the MSB at 1.
  83   - or better : select mapping for events
  84 What is the minimum granularity required ? (so we know how much LSB to cut)
  85   - How much can synchonized CPU TSCs drift apart one from another ?
  86     PLL
  87     http://en.wikipedia.org/wiki/Phase-locked_loop
  88     static phase offset -> tracking jitter
  89     25 MHz oscillator on motherboard for CPU
  90     jitter : expressed in ±picoseconds (should therefore be lower than 0.25ns)
  91     http://www.eetasia.com/ART_8800082274_480600_683c4e6b200103.HTM
  92     NEED MORE INFO.
  93   - What is the cacheline synchronization latency between the CPUs ?
  94     Worse case : Intel Core 2, Intel Xeon 5100, Intel core solo, intel core duo
  95     Unified L2 cache. http://www.intel.com/design/processor/manuals/253668.pdf
  96     Intel Core 2, Intel Xeon 5100
  97     http://www.intel.com/design/processor/manuals/253665.pdf
  98     Up to 10.7 GB/s FSB
  99     http://www.xbitlabs.com/articles/mobile/display/core2duo_2.html
 100                       Intel Core Duo     Intel Core 2 Duo
 101     L2 cache latency  14 cycles          14 cycles
 102     (round-trip : 28 cycles) 7ns @4GHz
 103     sparc64 : between threads : shares L1 cache.
 104     suspected to be ~2 cycles total (1+1) (to check)
 105   - How close (cycle-wise) can be two consecutive recorded events in the same
 106     buffer ? (~200ns, time for logging an event) (~800 cycles @4GHz)
 107   - Tracing code itself : if it's at a subbuffer boundary, more check to do.
 108     Must see the maximum duration of a non interrupted probe.
 109     Worse case (had NMIs enabled) : 6997 cycles. 1749 ns @4GHz.
 110     TODO : test with NMIs disabled and HT disabled.
 111     Ordering can be changed if an interrupt comes between the memory operation
 112     and the tracer call. Therefore, we cannot rely on more precision than the
 113     expected interrupt handler duration. (guess : ~10000cycles, 2500ns@4GHz)
 114   - If there is a faster interconnect between the CPUs, it can be a problem, but
 115     seems to only be proprietary interconnects, not used in general.
 116   - IPI are expected to take much more than 28 cycles.
 117 What is the minimum wrap-around interval ? (must be safe for timer interrupt
 118 miss and multiple timer HZ (configurable) and CPU MHZ frequencies)
 119 Must align _all_ headers on 32 bits, not 64.
 120
 121 Granularity : 800ns (200 cycles@4GHz) : 2^9 = 512 (remove 9 LSB)
 122   Number of LSB skipped : first_bit(probe_duration_in_cycles)-1
 123
 124 Min wrap : 100HZ system, each 3 timer ticks : 0.03s (32-4 MSB for 4 GHZ : 0.26s)
 125   (heartbeat each 100HZ, to be safe)
 126   Number of MSB to skip :
 127     32 - first_bit(( (expected_longest_cli()[ms] + max_timer_interval[ms]) * 2 /
 128                    cpu_khz ))
 129
 130
 131 9LSB + 4MSB = 13 bits total. 12 bits for event IDs : 4096 events.
 132
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