review request for 7142641: -Xshared:on fails on ARM
dean.long at oracle.com
Wed Feb 29 17:30:20 UTC 2012
Dan and David, thanks for the reviews.
On 2/28/2012 3:46 PM, Daniel D. Daugherty wrote:
> Wow! Ya learn something new every day!
> Thanks for the background info. And you can count me as
> a reviewer...
> Since change affects all Linuxes, you should move the
> bug to jvm/hotspot/runtime_cds...
OK will do.
> On 2/28/12 4:43 PM, Dean Long wrote:
>> Hi Dan,
>> yes, I was concerned about that as well. But MAP_SHARED isn't
>> actually needed to share read-only pages. If you trace an app's
>> startup using "strace" you can see that the dynamic linker maps
>> shared libraries such as libc with MAP_PRIVATE. Multiple processes
>> still share those pages through the page cache. Furthermore, the
>> linux mmap implementation turns off the shared flag if the file was
>> opened read-only (but unfortunately it turns off the flag only after
>> enforcing the cache coloring constraints.)
>> On 2/28/2012 2:40 PM, Daniel D. Daugherty wrote:
>>> I'm confused... I thought mapping the read-only CDS archive pages as
>>> "shared" permits those pages to be shared by other java processes that
>>> are also using the CDS archive...
>>> On 2/28/12 3:05 PM, Dean Long wrote:
>>>> Summary of changes: 3 lines changed: 0 ins; 2 del; 1 mod; 5538
>>>> The Class Data Sharing file is mapped "shared" on linux, which
>>>> places extra constraints on which virtual address a file offset can
>>>> use. This is to ensure consistency on certain platforms with
>>>> aliasing caches. However as the CDS file is opened read-only,
>>>> mapping it "shared" has no benefit and can cause the mmap to fail.
>>>> The simplest fix is to change the mapping to "private".
>>>> Tested on linux arm platform with aliasing cache. Installed CDS
>>>> with -Xshare:dump then ran through some vm tests as a sanity check.
>>>> This bug is on the 7u4 watch list.
More information about the hotspot-gc-dev