RFR: 8239129: Use DAX in ZGC

[Per Liden]

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