logging - Android logs 'GC_EXTERNAL_ALLOC' 'GC_FOR_MALLOC'

Question

While running my apps, I have got this kind of logs:

GC_EXTERNAL_ALLOC freed 2K, 38% free 8772K/14087K, external 17480K/17998K, paused 87ms
GC_FOR_MALLOC freed 0K, 38% free 8772K/14087K, external 17480K/17998K, paused 67ms
GC_CONCURRENT freed 2125K, 47% free 6214K/11719K, external 7142K/8400K, paused 3ms+5ms

Does anyone know what these logs mean? Thanks in advance!

1. What is the difference between 'GC_EXTERNAL_ALLOC', 'GC_FOR_MALLOC' and 'GC_CONCURRENT'. Are there some other 'GC' events?
2. What does '38% free 8772K/14087K' mean? What is '8772K' and what is '14087K'?
3. What does 'external 17480K/17998K' mean? What is '17480K' and what is '17998K'?

Thanks!

Answer 1

Another place where the Dalvik garbage collector messages are explained is in this video: Google I/O 2011: Memory management for Android Apps

At about 14 minutes into the presentation, he breaks down the message format. (BTW, that video has really good info on debugging memory leaks)

Roughly speaking, the format is [Reason] [Amount Freed], [Heap Statistics], [External Memory Statistics], [Pause Time]

Reason

Viktor/Robert already explained GC_CONCURRENT, GC_FOR_MALLOC, GC_EXTERNAL_ALLOC.

There is also:

• GC_HPROF_DUMP_HEAP - If you dump heap by clicking the "dump heap" button from DDMS or programatically

• GC_EXPLICIT - If you call System.gc()

Amount Freed

E.g. freed 2125K

Self explanatory

Heap Statistics

E.g. 47% free 6214K/11719K

These numbers reflect conditions after the GC ran. The "47% free" and 6214K reflect the current heap usage. The 11719K represents the total heap size. From what I can tell, the heap can grow/shrink, so you will not necessarily have an OutOfMemoryError if you hit this limit.

External Memory Statistics

E.g external 7142K/8400K

Note: This might only exist in pre-Honeycomb versions of Android (pre 3.0).

Before Honeycomb, bitmaps are allocated external to your VM (e.g. Bitmap.createBitmap() allocates the bitmap externally and only allocates a few dozen bytes on your local heap). Other examples of external allocations are for java.nio.ByteBuffers.

Pause Time

If it's a concurrent GC event, there will be two times listed. One is for a pause before the GC, one is for a pause when the GC is mostly done. E.g. paused 3ms+5ms

For non-concurrent GC events, there is only one pause time and it's typically much bigger. E.g. paused 87ms