review request for 7142641: -Xshared:on fails on ARM
dean.long at oracle.com
Tue Feb 28 15:43:16 PST 2012
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
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 unchg
>> 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.
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