[9] RFR(M): 8029799: vm/mlvm/anonloader/stress/oome prints warning: CodeHeap: # of free blocks (original) (raw)

Albert albert.noll at oracle.com
Wed Feb 5 08:28:47 PST 2014

Hi Vladimir,

thanks for looking at this. I've done the proposed measurements. The code which I used to get the data is included in the following webrev:

http://cr.openjdk.java.net/~anoll/8029799/webrev.01/

I think some people might be interested in getting that data, so we might want to keep that additional output. The exact output format can be changed later (JDK-8005885).

Here are the results:

The minimum size that can be allocated from the code cache is platform-dependent. I.e., the minimum size depends on CodeCacheSegmentSize and CodeCacheMinBlockLength. On x86, for example, the min. allocatable size from the code cache is 64*4=256bytes. The size of adapters ranges from 400b to 600b. Here is the beginning of the nmethod size distribution of the failing test case:

nmethod size distribution (non-zombie java)

0-16 bytes 0[bytes] 16-32 bytes 0 32-48 bytes 45 48-64 bytes 0 64-80 bytes 41 80-96 bytes 0 96-112 bytes 6247 112-128 bytes 0 128-144 bytes 249 144-160 bytes 0 160-176 bytes 139 176-192 bytes 0 192-208 bytes 177 208-224 bytes 0 224-240 bytes 180 240-256 bytes 0 ...

I do not see a problem for increasing the CodeCacheSegmentSize if tiered compilation is enabled.

What do you think?

Best, Albert

On 02/04/2014 05:52 PM, Vladimir Kozlov wrote:

I think the suggestion is reasonable since we increase CodeCache *5 for Tiered. Albert, is it possible to collect data how much space is wasted in % before and after this change: free space in which we can't allocate + unused bytes at the end of nmethods/adapters? Can we squeeze an adapter into 64 bytes?

Thanks, Vladimir On 2/4/14 7:41 AM, Albert wrote: Hi,

could I get reviews for this patch (nightly failure)? webrev: http://cr.openjdk.java.net/~anoll/8029799/webrev.00/ bug: https://bugs.openjdk.java.net/browse/JDK-8029799 problem: The freelist of the code cache exceeds 10'000 items, which results in a VM warning. The problem behind the warning is that the freelist is populated by a large number of small free blocks. For example, in failing test case (see header), the freelist grows up to more than 3500 items where the largest item on the list is 9 segments (one segment is 64 bytes). That experiment was done on my laptop. Such a large freelist can indeed be a performance problem, since we use a linear search to traverse the freelist. solution: One way to solve the problem is to increase the minimal allocation size in the code cache. This can be done by two means: we can increase CodeCacheMinBlockLength and/or CodeCacheSegmentSize. This patch follows the latter approach, since increasing CodeCacheSegmentSize decreases the size that is required by the segment map. More concretely, the patch doubles the CodeCacheSegmentSize from 64 byte to 128 bytes if tiered compilation is enabled. The patch also contains an optimization in the freelist search (stop searching if we found the appropriate size) and contains some code cleanups. testing: With the proposed change, the size of the freelist is reduced to 200 items. There is only a slight increase in memory required by code cache by at most 3% (all data measured for the failing test case on a Linux 64-bit system, 4 cores). To summarize, increasing the minimum allocation size in the code cache results in potentially more unused memory in the code cache due to unused bits at the end of an nmethod. The advantage is that we potentially have less fragmentation. proposal: - I think we could remove CodeCacheMinBlockLength without loss of generality or usability and instead adapt the parameter CodeCacheSegmentSize at Vm startup. Any opinions? Many thanks in advance, Albert

More information about the hotspot-compiler-dev mailing list