> On Sep 16, 2019, at 7:50 AM, Roman Simakov <[email protected]> wrote:
>
> Hi,
>
> I guess it would be interesting alternative for spinlock
> (https://kernelnewbies.org/Linux_5.3)
> 1.6. Power efficient userspace waiting with the umwait x86 instructions
>
> More description is here:
> https://lwn.net/Articles/790920/
>
Spinlocks in userspace are a performance disaster - here are some rhoughts from
Jim Starkey ...
Spin locks were invented at DEC for use in either the RSX-11M+ or VMS kernel
for a very specific and narrow purpose. Spin locks were used when the OS was
initiating a very short device request where the expected completion time was
less than the overhead to setup for and process a device interrupt. They were
used sparingly and effectively.
For virtually any other purpose, spin locks are a huge net loss for a very
simple reason: Hogging the processor waiting for another thread or process not
only wastes processor cycles but a quite likely to present the other process
and thread from running and releasing whatever resource had induced the spin.
The last great performance leap on the Falcon/InnoDB race at MySQL was when
Google replaced InnoDB's spin locks with Falcon's user mode synchronization
locks, giving InnoDB a 20% or 30% kick under heavy load.
Long (or indeterminate) locks are best handled with user mode read/write
synchronization objects. Very short term locks can be avoided through
judicious use of non-interlocked data structures managed with compare-and-swap.
AmorphousDB, for example, uses non-interlocked hash tables for things like
transactions and network operation tickets; objects in these hash tables can be
safely removed by first moving them into an object purgatory that is safely
emptied by a cycle manager once a second (or whatever).
There is an incredible among of non-sense written about spin locks by folks who
tend to believe that any technique used by and OS kernel must be very cool and
very efficient.
The related idea of an instruction that can stall a processor waiting on memory
write is every bit as flawed as spin locks even if it does reduce the amount of
power consumed while gumming up the OS scheduler.
--
Jim Starkey
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