valgrind-users

[Valgrind-users] False Sharing Detection

[Ivan N. <nov...@gm...>]

Hi,
I would like to detect cases of false sharing in my program.

By this I mean cases where multiple threads are both trying to write data to
different memory locations but those locations are closest enough in memory
that they are on the on the same cache line.

The result would be delays on threads even though there are enough CPU's to
do the work.

Are there any tools in valgrind to do this?

Regards,
Ivan Novick

Re: [Valgrind-users] False Sharing Detection

[Nicholas N. <n.n...@gm...>]

On Thu, May 28, 2009 at 9:12 AM, Ivan Novick <nov...@gm...> wrote:
> Hi,
> I would like to detect cases of false sharing in my program.
> By this I mean cases where multiple threads are both trying to write data to
> different memory locations but those locations are closest enough in memory
> that they are on the on the same cache line.
> The result would be delays on threads even though there are enough CPU's to
> do the work.
> Are there any tools in valgrind to do this?

No, but it wouldn't be too hard to write a new one that did this.  For
each cache-line-sized chunk of memory, you'd record which thread last
wrote to it and when, and then if another thread wrote to a chunk
sufficiently soon after a previous thread you'd give a warning.  You
could record thread numbers and addresses and stack traces and the
like, there'd be trade-offs in how much overhead you'd have vs. how
much info you'd get in each report.

Nick

[Valgrind-users] Fwd: False Sharing Detection

[Ivan N. <nov...@gm...>]

---------- Forwarded message ----------
From: Ivan Novick <nov...@gm...>
Date: Wed, May 27, 2009 at 4:52 PM
Subject: Re: [Valgrind-users] False Sharing Detection
To: Nicholas Nethercote <n.n...@gm...>


A report that would be very help full for application developers would be
like this:
LINE OF CODE                        # of INSTANCES OF FALSE SHARING
DETECTED
array[x] = val;  main.c:44           1,234,567
b = y; main.c:124                      134,233
k = l; sum.c:233                        44,123
...... etc

To help app developers quickly find the biggest violators

Regards,
Ivan Novick


On Wed, May 27, 2009 at 4:45 PM, Nicholas Nethercote <n.n...@gm...
> wrote:

> On Thu, May 28, 2009 at 9:12 AM, Ivan Novick <nov...@gm...> wrote:
> > Hi,
> > I would like to detect cases of false sharing in my program.
> > By this I mean cases where multiple threads are both trying to write data
> to
> > different memory locations but those locations are closest enough in
> memory
> > that they are on the on the same cache line.
> > The result would be delays on threads even though there are enough CPU's
> to
> > do the work.
> > Are there any tools in valgrind to do this?
>
> No, but it wouldn't be too hard to write a new one that did this.  For
> each cache-line-sized chunk of memory, you'd record which thread last
> wrote to it and when, and then if another thread wrote to a chunk
> sufficiently soon after a previous thread you'd give a warning.  You
> could record thread numbers and addresses and stack traces and the
> like, there'd be trade-offs in how much overhead you'd have vs. how
> much info you'd get in each report.
>
> Nick
>