Re: [Xen-devel] Re: NUMA and SMP

[tgh <tianguanhua@xxxxxxxxxx>]

I am puzzled ,what is the page migration?
Thank you in advance


Emmanuel Ackaouy åé:
> On the topic of NUMA:
>
> I'd like to dispute the assumption that a NUMA-aware OS can actually
> make good decisions about the initial placement of memory in a
> reasonable hardware ccNUMA system.
>
> How does the OS know on which node a particular chunk of memory
> will be most accessed? The truth is that unless the application or
> person running the application is herself NUMA-aware and can provide
> placement hints or directives, the OS will seldom beat a round-robin /
> interleave or random placement strategy.
>
> To illustrate, consider an app which lays out a bunch of data in memory
> in a single thread and then spawns worker threads to process it.
>
> Is the OS to place memory close to the initial thread? How can it 
> possibly
> know how many threads will eventually process the data?
>
> Even if the OS knew how many threads will eventually crunch the data,
> it cannot possibly know at placement time if each thread will work on an
> assigned data subset (and if so, which one) or if it will act as a 
> pipeline
> stage with all the data being passed from one thread to the next.
>
> If you go beyond initial memory placement or start considering memory
> migration, then it's even harder to win because you have to pay copy
> and stall penalties during migrations. So you have to be real smart
> about predicting the future to do better than your ~10-40% memory
> bandwidth and latency hit associated with doing simple memory
> interleaving on a modern hardware-ccNUMA system.
>
> And it gets worse for you when your app is successfully taking advantage
> of the memory cache hierarchy because its performance is less impacted
> by raw memory latency and bandwidth.
>
> Things also get more difficult on a time-sharing host with competing
> apps.
>
> There is a strong argument for making hypervisors and OSes NUMA
> aware in the sense that:
> 1- They know about system topology
> 2- They can export this information up the stack to applications and 
> users
> 3- They can take in directives from users and applications to 
> partition the
> host and place some threads and memory in specific partitions.
> 4- They use an interleaved (or random) initial memory placement strategy
> by default.
>
> The argument that the OS on its own -- without user or application
> directives -- can make better placement decisions than round-robin or
> random placement is -- in my opinion -- flawed.
>
> I also am skeptical that the complexity associated with page migration
> strategies would be worthwhile: If you got it wrong the first time, what
> makes you think you'll do better this time?
>
> Emmanuel.
>
>
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