Nick Piggin wrote:
Brian Twichell wrote:
If we had to choose between pagetable sharing for small pages and
hugepages, we would be in favor of retaining pagetable sharing for
small pages. That is where the discernable benefit is for customers
that run with "out-of-the-box" settings. Also, there is still some
benefit there on x86-64 for customers that use hugepages for the
bufferpools.
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%.
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.
Cheers,
Brian
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