CMPXCHG latency

By: Linus Torvalds (torvalds.delete@this.linux-foundation.org), March 31, 2008 10:16 am
Room: Moderated Discussions
Michael S (already5chosen@yahoo.com) on 3/31/08 wrote:
>
>Non-contended spinlock doesn't mean the absence of cache-
>bouncing. I'd guess that in majority of practical cases you
>will see a combination of very low contention rate with
>high (or, when the number of threads exceeds two, very
>high) cache-bouncing rates.

There are lots of very common cases where you have
absolutely no cache contention or bouncing at all in
practice.

Any piece of code that is thread-safe has to have locking,
but a lot of it is then not used with threads at all, or
has the threads idle - waiting for some event.

This includes some very basic code like core library code,
and under many loads that code is hot and critical. Think
about something as fundamental as just "malloc()", and
think about the fact that the same library code is used
for both threaded and non-threaded applications.

And even for things like a kernel, and with hundreds of
threads active, it's quite common that you have lots of
very core data structures that might be touched by
multiple threads, but often are not in practice.

Things like locks for individual files or directories, or
a socket. Yeah, you may have multiple threads or processes
operating on them, but in 99% of all cases there is just a
single thread doing it - even when there are other threads
touching other files.

Some code goes to extreme extents to avoid the locking
costs, by turning it into a code size cost instead (ie
using conditional code to avoid the actual lock when you
know it's not needed). That doesn't always work (ie a lock
may not have any dynamic contention, but it could be
reached in theory, and has its own costs, obviously.

Linus
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