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

From: Daniel Walker
Date: Thu Mar 29 2012 - 13:10:03 EST

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

> 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:

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

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.

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