Speed up trace mode (GSoC Project)

[Clemens Lang]

Hi,

On Wed, May 01, 2019 at 11:57:14AM +0530, Mihir Luthra wrote:
> For making my implementation of shared memory data structure more
> space efficient, I was trying to implement a stack which stores
> offsets to unused locations in the shared memory file. But as stack is
> being shared it also needs to be edited in a lock free way. While
> editing stack I need to atomically CAS both top of stack and the
> element on it. For this I found double compare and swap. Also, in my
> data structure at one point I need DCAS as well to ensure correct
> editing.
> To implement DCAS I came across some instruction “cmpxchg16”. But I
> think its still not as per my need. [1]
> 
> Do you know any alternative with which I can DCAS atomically or
> anything which atomically checks 2 old values before replacing value
> at an address?
> 
> [1] https://stackoverflow.com/questions/7646018/sse-instructions-which-cpus-can-do-atomic-16b-memory-operations

>From what I understand from the stackoverflow post you're right that
cmxpchg16b will not give a consistent view of the 16 bytes of memory
across multiple NUMA nodes. However, maybe two 4 byte values right next
to each other would be sufficient for your use case and could then be
casted to a 8 byte values for CAS?

-- 
Clemens