4 ms·

Are any of these a surprise to anyone here? I am not asking this rhetorically.

by yvdriess 12y ago

ohashi 12y ago
It was definitely interesting to learn a few little things like 1 divide being slower than +-
p1esk 12y ago
For me, the surprise was to learn that access time for L1 cache is 3 cycles. I thought it should be 1 cycle.
- Tuna-Fish 12y ago
  Actually 4 cycles for modern cpus. The access time of a cache depends mostly on it's size (wire delay and switching time for the access tree). This means that cache sizes are always tradeoffs between hit rate and access time. As cpus get better at hiding latency, the access latency is loosened.
wtracy 12y ago
The idea that mutating files in place is actually very expensive is new to me.
- deleted 12y ago
  [deleted]
t1m 12y ago
I found the SSD discussion very useful. Bottom line is that deleting (or rewriting) data causes unpredictable and degraged write performance due to "write amplification". Sticking to appends-only writes keeps things fast. I certainly wouldn't discourage anyone from watching the video. At least read the slides! http://gotocon.com/dl/goto-aar-2012/slides/MartinThompson_MythbustingModernHardwareToGainMechanicalSympathy.pdf http://gotocon.com/dl/goto-aar-2012/slides/MartinThompson_My...
gizmo686 12y ago
Network cards write directly into cache
- hhw 12y ago
  This feature, known as DDIO, is only on Intel's Xeon E5 and E7 v2 family CPUs. So, it's not quite ubiquitous yet, nor have there been any announcements on when it will become available (if ever) on the lower Xeon E3 or any of the desktop lines, despite a good amount of information already having been revealed on the next 2-3 generations of processors.
  - neckbeard 12y ago
    DCA (Direct Cache Access) has been around for a while - DDIO is its replacement on higher end Xeons.
mtdewcmu 12y ago
I was a little surprised that it was called a myth that CPUs are not getting faster. CPUs may still be getting a little faster, but they are way off the exponential curve that they were on at one time. They look like they're asymptotically approaching some constant now. It certainly wouldn't hurt your code at all if you make the assumption that CPUs are no longer getting faster. Your code would probably be better.
- InclinedPlane 12y ago
  CPUs are still getting a lot faster, at a geometric rate. But they are doing so at a different rate than they did in the '90s, where CPUS were getting enormously faster year over year. Here's a chart (note that the vertical scale is logarithmic): http://preshing.com/images/integer-perf.png http://preshing.com/images/integer-perf.png In the '90s processors were doubling in performance every 20 months or so (i.e. a factor of 64x per decade). Since about 2005 processors have been doubling in performance about every 43 months. This is much slower than the insanely fast rate during the '90s but it is still very fast and still on an exponential curve.
  - lloeki 12y ago
    > Here's a chart Interesting that in order to compare apples to apples it's single threaded. That means for a back of the envelope estimation of ideally parallel loads performance, the graph goes "through the roof" since it's exactly twice on my laptop or four times on my desktop.
    - cma 12y ago
      The graph is logarithmic, so it might not go entirely through the roof.
  - mtdewcmu 12y ago
    Why does it stop at 2011? The last that I've heard of Haswell was that it was a paltry improvement, and that core will be around for the next couple years.

Are any of these a surprise to anyone here? I am not asking this rhetorically.

ohashi 12y ago

It was definitely interesting to learn a few little things like 1 divide being slower than +-

p1esk 12y ago

For me, the surprise was to learn that access time for L1 cache is 3 cycles. I thought it should be 1 cycle.

Tuna-Fish 12y ago
Actually 4 cycles for modern cpus. The access time of a cache depends mostly on it's size (wire delay and switching time for the access tree). This means that cache sizes are always tradeoffs between hit rate and access time. As cpus get better at hiding latency, the access latency is loosened.

wtracy 12y ago

The idea that mutating files in place is actually very expensive is new to me.

deleted 12y ago
[deleted]

t1m 12y ago

I found the SSD discussion very useful. Bottom line is that deleting (or rewriting) data causes unpredictable and degraged write performance due to "write amplification". Sticking to appends-only writes keeps things fast. I certainly wouldn't discourage anyone from watching the video. At least read the slides! http://gotocon.com/dl/goto-aar-2012/slides/MartinThompson_MythbustingModernHardwareToGainMechanicalSympathy.pdf http://gotocon.com/dl/goto-aar-2012/slides/MartinThompson_My...

gizmo686 12y ago

Network cards write directly into cache

hhw 12y ago
This feature, known as DDIO, is only on Intel's Xeon E5 and E7 v2 family CPUs. So, it's not quite ubiquitous yet, nor have there been any announcements on when it will become available (if ever) on the lower Xeon E3 or any of the desktop lines, despite a good amount of information already having been revealed on the next 2-3 generations of processors.
- neckbeard 12y ago
  DCA (Direct Cache Access) has been around for a while - DDIO is its replacement on higher end Xeons.

mtdewcmu 12y ago

I was a little surprised that it was called a myth that CPUs are not getting faster. CPUs may still be getting a little faster, but they are way off the exponential curve that they were on at one time. They look like they're asymptotically approaching some constant now. It certainly wouldn't hurt your code at all if you make the assumption that CPUs are no longer getting faster. Your code would probably be better.

InclinedPlane 12y ago
CPUs are still getting a lot faster, at a geometric rate. But they are doing so at a different rate than they did in the '90s, where CPUS were getting enormously faster year over year. Here's a chart (note that the vertical scale is logarithmic): http://preshing.com/images/integer-perf.png http://preshing.com/images/integer-perf.png In the '90s processors were doubling in performance every 20 months or so (i.e. a factor of 64x per decade). Since about 2005 processors have been doubling in performance about every 43 months. This is much slower than the insanely fast rate during the '90s but it is still very fast and still on an exponential curve.
- lloeki 12y ago
  > Here's a chart Interesting that in order to compare apples to apples it's single threaded. That means for a back of the envelope estimation of ideally parallel loads performance, the graph goes "through the roof" since it's exactly twice on my laptop or four times on my desktop.
  - cma 12y ago
    The graph is logarithmic, so it might not go entirely through the roof.
- mtdewcmu 12y ago
  Why does it stop at 2011? The last that I've heard of Haswell was that it was a paltry improvement, and that core will be around for the next couple years.