4 ms·

If you add -XX:+AlwaysPreTouch to the JVM arguments, the JVM will pre-touch the entirety of the heap at startup time to avoid unpredictable page faults through

If you add -XX:+AlwaysPreTouch to the JVM arguments, the JVM will pre-touch the entirety of the heap at startup time to avoid unpredictable page faults through the life of the application. I'd imagine other HFT companies may also pay for the Azul JVM which goes even further and comes with a kernel module that the JVM coordinates with for memory allocation. The kernel module pre-reserves x% of system memory at the time it is loaded.

nobleach 6y ago
Is the Azul JVM open source? I thought it was just the support which costed money. (I'm just not familiar enough with it)
- whizzter 6y ago
  It is commercial, and sadly their GC algorithms are patented. I'm really anxious for those patents to start running out because the functionality is quite clever. Basically one main reason you need to stop the world in common GC's right now (and thus cause pauses) is because if you want to compact memory a GC thread has no idea if other threads reads/writes to an object while moving leading to worst case corrupt/lost writes. What Azul does is to make the pages in question invalid for reading/writing so any access to them would page-fault and then proceeds to move the objects without worrying about other threads. Since the hardware provides the page-fault "for-free" this protection doesn't cost anything in terms of runtime performance in the common case (compared to the new ZGC made by Oracle that has read-write barriers that steals some mutator performance), worst case if another thread tries to read/write memory that is being moved a specialized page-fault handler detects if this was an page that was in motion and can then do a slower read-write but ONLY in the seldom cases where this occurs (compared to all the time of f.ex. ZGC)
  - abledon 6y ago
    why does the hardware provide page-fault for free again?
    - whizzter 6y ago
      Page-faults aren't for free (actually kinda expensive) but rather the paging functionality is giving "free" read/write-barriers instead of those being instruction sequences being inside the main program (ie read/write barriers are handled as page-faults by the paging system that the OS already provides to avoid requiring every read/write of the user-code to be aware of moving pointers).
  - nobleach 6y ago
    Hey thanks. That was a fantastic explanation.
- shemnon42 6y ago
  There's two, Zing (pay for support) and Zulu (pay for access)

nobleach 6y ago

Is the Azul JVM open source? I thought it was just the support which costed money. (I'm just not familiar enough with it)

whizzter 6y ago
It is commercial, and sadly their GC algorithms are patented. I'm really anxious for those patents to start running out because the functionality is quite clever. Basically one main reason you need to stop the world in common GC's right now (and thus cause pauses) is because if you want to compact memory a GC thread has no idea if other threads reads/writes to an object while moving leading to worst case corrupt/lost writes. What Azul does is to make the pages in question invalid for reading/writing so any access to them would page-fault and then proceeds to move the objects without worrying about other threads. Since the hardware provides the page-fault "for-free" this protection doesn't cost anything in terms of runtime performance in the common case (compared to the new ZGC made by Oracle that has read-write barriers that steals some mutator performance), worst case if another thread tries to read/write memory that is being moved a specialized page-fault handler detects if this was an page that was in motion and can then do a slower read-write but ONLY in the seldom cases where this occurs (compared to all the time of f.ex. ZGC)
- abledon 6y ago
  why does the hardware provide page-fault for free again?
  - whizzter 6y ago
    Page-faults aren't for free (actually kinda expensive) but rather the paging functionality is giving "free" read/write-barriers instead of those being instruction sequences being inside the main program (ie read/write barriers are handled as page-faults by the paging system that the OS already provides to avoid requiring every read/write of the user-code to be aware of moving pointers).
- nobleach 6y ago
  Hey thanks. That was a fantastic explanation.
shemnon42 6y ago
There's two, Zing (pay for support) and Zulu (pay for access)