Messages in this thread | | | Date | Fri, 12 May 2006 15:17:38 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 0/2][RFC] New version of shared page tables |
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Brian Twichell wrote: > Nick Piggin wrote:
>> Of course if it was free performance then we'd want it. The downsides >> are that it >> is a significant complexity for a pretty small (3%) performance gain >> for your apparent >> target workload, which is pretty uncommon among all Linux users. > > > Our performance data demonstrated that the potential gain for the > non-hugepage case is much higher than 3%.
The point is, there are hugepages. They were a significant additional complexity but the concession was made because they did provide a large speedup for databases.
> >> >> Ignoring the complexity, it is still not free. Sharing data across >> processes adds to >> synchronisation overhead and hurts scalability. Some of these page >> fault scalability >> scenarios have shown to be important enough that we have introduced >> complexity _there_. > > > True, but this needs to be balanced against the fact that pagetable > sharing will reduce the number of page faults when it is achieved. > Let's say you have N processes which touch all the pages in an M page > shared memory region. Without shared pagetables this requires N*M page > faults; if pagetable sharing is achieved, only M pagefaults are required. > >> >> And it seems customers running "out-of-the-box" settings really want >> to start using >> hugepages if they're interested in getting the most performance >> possible, no? > > > My perspective is that, once the customer is required to invoke "echo > XXX > /proc/sys/vm/nr_hugepages" they've left the "out-of-the-box" > domain, and entered the domain of hoping that the number of hugepages is > sufficient, because if it's not, they'll probably need to reboot, which > can be pretty inconvenient for a production transaction-processing > application.
I think it is pretty easy to reserve hugepages at bootup. This is what a production transaction processing system will be doing, won't it? Especially if they're performance constrained and hugepages gives them a 30% performance boost.
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