Questions around codeheaps, codeblobs and code locality

[Tobias Hartmann]

Hi,

> On 6/3/19 12:14 PM, keita abdoul-kader wrote:
>> I have been looking into the code cache layout and ways to reduce the high
>> amount of iTLB we are observing on some of our workloads. It seems that
>> after the works on segmented codeHeaps, the plan was to add support for non
>> contiguous codeBlobs as the next step to improve code locality in the code
>> cache. is the work  on non contiguous codeBlobs still active ?

With the segmented code cache, we've seen some nice improvements (slide 19 onwards):
http://cr.openjdk.java.net/~thartmann/talks/2014-Efficient_Code_Cache_Management.pdf

We had several ideas on how to improve things further (see slide 31) but there are currently no
plans to work on any of these.

>> Also, does the hotspot has support for any form of code compaction ?
>> Compacting the code heap is mentioned in one of the comment in
>> relocInfo.hpp, but browsing the source code it seems that once  the
>> codeBlobs is filled with the content of a codeBuffer, they are never moved
>> in a way to reduce codeheap fragmentation.

No, code in the code cache is not moved/compacted but only removed.

>>   In general, is there an umbrella project or a set of open task around
>> improving code locality ?  Please note that most of my observation driving
>> this come from spark-sql workloads. Since spark-sql generates dynamically
>> generate a lots of code, i might simply be observing a pathological case.

No, we don't have any umbrella project for this.

Best regards,
Tobias