Re: [RFC] Android Logger vs. Shared Memory FIGHT!

[Daniel Walker <dwalker@fifo99.com>]

On Thu, Mar 29, 2012 at 09:25:26AM -0700, Tim Bird wrote:
> 
> There is concern about applications leaking sensitive data into the logs,
> and a desire to possibly (in the future) support per-application logs
> for some apps.  Having the code in-kernel means that things like the
> timestamp, tid and pid cannot be forged by the process.  The separation
> into channels and kernel management of the read/write position provide
> an impediment to denial of service attacks.

it's unclear what problem would be caused by forging a PID, and why any
app would want to do that. It doesn't look like it would cause any
problems.

> At the moment, I'm not considering an alternative for logger that runs
> completely in user-space.  Having said that, this test is certainly interesting,
> and may provide some performance numbers for logger or alternatives that would
> be useful to compare.

It's certainly up to you what you want to use, but I don't see why any
other architecture or OS would want to use logger.
 
> I like that you've put the gettimeofday() into the shared memory test, to
> capture the cost of the timestamp operation.  Presumably, the fact that
> x86 has VDSO and ARM does not is contributing to the performance difference
> between the two platforms.

John Stultz commented on how gettimeofday() is more accurate that what
is needed. Logger actually uses a less accurate, and faster, method of timing.
The time isn't exactly apples to apples, my shared memory example is actually
using a slower clock compared to logger.
 
> 
> Tests were 60 seconds.  I presume there were multiple runs and these are
> averages.  Can you provide the number of runs and the standard deviation for
> each set?


Test cases were 600 seconds (10 minutes) .. There were three runs. 
Here's the raw data, all in bytes per second:

X86:
Logger TSC = 70072480.0, 83911562.7, 78610971.8
Shared Memory TSC = 460262336.0, 454204684.1, 457050682.2

Logger ACPI_PM = 75523320.0, 81615111.9, 81588600.2
Shared Memory ACPI_PM = 29688532.0, 28211029.2, 28225661.6


ARM:
Logger = 6985508.0, 5827587.3, 7034293.7
Shared Memory = 12371886.7, 18362639.9, 15274489.3

You can feel free to compute the standard deviation if you wish.

Daniel