[boost] Re: linked_ptr update (long)

From: Greg Colvin (gcolvin_at_[hidden])
Date: 2000-02-08 01:39:59

With a spin count of 4000 I wonder why they bother with the
system call at all -- that's a lot of contention.

----- Original Message ----- 
From: Andy Sawyer <boost_at_[hidden]>
To: <boost_at_[hidden]>
Sent: Monday, February 07, 2000 10:40 PM
Subject: [boost] Re: linked_ptr update (long)

(Sorry, this is all VERY Win32 specific!)

On 08 February 2000 01:40, Borgerding, Mark A.wrote:

> Re: CRITICAL_SECTION
> I think the Win32 critical section mechanism basically uses
> spinlocks.  The struct definition (in winnt.h) has a DWORD member
> called SpinCount.

 Not quite - from the documentation for the API call 
  InitializeCriticalSectionAndSpinCount (which arrived with NT4SP3 and Win98),
we get:

 "The spin count is useful for critical sections of short duration that can
  experience high levels of contention. Consider a worst-case scenario, in
  which an application on an SMP system has two or three threads constantly
  allocating and releasing memory from the heap. The application serializes
  the heap with a critical section. In the worst-case scenario, contention
  for the critical section is constant, and each thread makes an expensive
  call to the WaitForSingleObject function. However, if the spin count is
  set properly, the calling thread will not immediately call
  WaitForSingleObject when contention occurs. Instead, the calling thread
  can acquire ownership of the critical section if it is released during the
  spin operation.

  You can improve performance significantly by choosing a small spin count
  for a critical section of short duration. The heap manager uses a spin
  count of roughly 4000 for its per-heap critical sections. This gives great
  performance and scalability in almost all worst-case scenarios."

 As I read it, a Win32 critical section will spin for a determined time
before executing the Wait. So it's actually half-and-half :)

Regards,
 Andy
-- 
Andy Sawyer, Technical Director, Sufficiently Advanced Technology Ltd.
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