RFR: 8239129: Use DAX in ZGC

Stefan Karlsson stefan.karlsson at oracle.com
Fri Feb 21 11:30:18 UTC 2020

Looks good.


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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