RFR: 8239129: Use DAX in ZGC

Yasumasa Suenaga suenaga at oss.nttdata.com
Wed Feb 19 08:43:53 UTC 2020

Hi Per,

Thanks for updating JBS and for creating patch!
Your change looks good to me.
Please list me as Reviewer.



On 2020/02/19 17:07, 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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