RFR: 8239129: Use DAX in ZGC

Per Liden per.liden at oracle.com
Mon Feb 24 10:53:03 UTC 2020

Thanks Stefan!


On 2/21/20 12:30 PM, Stefan Karlsson wrote:
> Looks good.
> StefanK
> On 2020-02-19 09:07, Per Liden wrote:
>> On 2/17/20 1:28 PM, Yasumasa Suenaga wrote:
>> [...]
>>>>>>>    webrev: 
>>>>>>> http://cr.openjdk.java.net/~ysuenaga/JDK-8239129/webrev.00/
>>>>>> Before this patch can go forward, you need to get to the bottom of 
>>>>>> how to get that ioctl command to work. If it's not possible, you 
>>>>>> need to explain why and propose alternatives that we can discuss.
>>>>> I guess it is caused by Linux kernel.
>>>>> In case of ext4, `ext4_iflags_to_xflags()` would set filesystem 
>>>>> flags to `struct FS_IOC_FSGETXATTR`.
>>>>> However `FS_XFLAG_DAX` is not handled in it.
>>>> Did a bit of googleing and it seems the DAX flag is in a bit of flux 
>>>> at the moment. I guess this will be fixed down the road, when DAX in 
>>>> the kernel becomes a non-experimental feature.
>>>> How about we just do like this for now:
>>>> http://cr.openjdk.java.net/~pliden/8239129/webrev.0
>>> I thought ZGC requires tmpfs or hugetlbfs due to performance reason.
>>> So I introduced new -XX option to make users aware of it.
>> The filesystem type check is there to help users avoid the mistake of 
>> placing the heap on an unintended/slow filesystem. However, most users 
>> will never use -XX:AllocateHeapAt, so I think that risk is fairly 
>> small to begin with.
>> The bar for adding new options to ZGC is high, and I don't think it's 
>> high enough in this case. Also, other GCs happily allow you to place 
>> the heap on any filesystem and I don't mind having that flexibility in 
>> ZGC too.
>>> If not so, I agree with your change.
>> Ok, thanks.
>> I updated the patch, added and adjusted some logging, and added a 
>> test. I also updated the bug title/description.
>> http://cr.openjdk.java.net/~pliden/8239129/webrev.1
>> cheers,
>> Per

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