Re: MT allocator?

[Phil Edwards <phil at jaj dot com>]

On Sat, Jun 01, 2002 at 12:58:41PM +0200, Stefan Olsson wrote:
> In the documentation under "Available allocators in namespace std" there 
> is a note "[Another threadsafe allocator where each thread keeps its own 
> free list, so that no locking is needed, might be described here.]".

That was sort of a reminder to myself to come back and clean up the
pthread_alloc header mess.  And once that was done, document the result.


> Is anybody working on such an allocator? The issue with the default 
> allocator in a threaded enviorment is as you know that it locks the 
> entire heap and therefore seriously degrades the performance (i.e. we 
> are doing a lot of string operations, putting strings into vectors and 
> maps in each thread etc).

Right.  Thus pthread_alloc...


> However we cannot use the pthread_alloc implementation (from SGI) since 
> we are also maintaining an application wide hash_map of objects for 
> caching purposes (i.e. when one thread has completed a specific request, 
> that result is copied into the hash_map and when another thread modifies 
> data that was part of building the cached result this record is removed 
> from the cache by that thread). This means that the thread deleting 
> objects from the hash_map will take ownership for that memory...

I'm confused by the last sentence; is that the behavior you want, or the
behavior you don't want?


> One solution would be to have both global and thread specific free 
> lists, such as the implementation at 
> http://www.bti.net/cgi-bin/bti/show_header.pl?file=bti_thread_alloc.h

That does look interesting, although I'm instinctively frightened by user
code that declares all its stuff inside namespace std...


> When thinking about this, the next question would be how to improve "per 
> thread locality" and this is _really_ new stuff to me, but from what I 
> have read so far can make a huge difference on MP machines...

Yep.  From what I've heard, half the C++ community is pressuring the
committee to add threading support, and the other half is pressuring them
to leave it out (because if the committee specifies something stupid,
everyone loses).


Phil

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