4 ms·

malloc takes out a global lock and performance doesn't 'grind to a halt' from a single allocation. Hundreds of thousands of allocations per second per core and

malloc takes out a global lock and performance doesn't 'grind to a halt' from a single allocation. Hundreds of thousands of allocations per second per core and concurrency will suffer, but most programs either don't actually have nearly enough concurrency or minimize their allocations to the point where it isn't a primary bottleneck.

throwaway17_17 7y ago
I think the context you’re working in matters heavily with such an assertion. In fact, it is quite common in game development to bulk allocate and then use custom allocators within these regions specifically avoid the drag on performance and determinism inflicted by malloc.
- BubRoss 7y ago
  I don't think this confronts anything I said. Bulk allocations with custom allocators would either be exactly what I said (low malloc calls) or would just not use malloc at all and use the system memory mapping functions. The larger point was that an exception being thrown and taking a global lock is not going to tank performance. That would imply that all other threads are trying to take the same lock at the same time and that the thread that gets it holds it for a long time. Even in the case of malloc, where there could be actual heavy lock contention, this is not always a bottleneck.
zabzonk 7y ago
> malloc takes out a global lock Says who? The C Standard says nothing of the kind.
- BubRoss 7y ago
  All modern common implementations (I think). MSVC's malloc definitely locks. I think gcc, clang and icc do too. This is a big reason why tcmalloc and jemalloc were created.
  - gpderetta 7y ago
    I don't know about MSVC, but glibc malloc has per thread caches. At some point you need to hit the global allocator or sbrk or mmap of course and that might take a global lock.
lovecg 7y ago
Of course, it would depend on the workload. An exception here and there wouldn’t matter, but for a somewhat contrived example, using exceptions for something like a bad connection to a database on a server with dozens of threads could easily turn into a concurrency and a resource exhaustion problem (where every thread starts seeing the same error at the same time).
jcelerier 7y ago
> malloc takes out a global lock not anymore for a couple years with glibc : https://www.phoronix.com/scan.php?page=news_item&px=glibc-malloc-thread-cache https://www.phoronix.com/scan.php?page=news_item&px=glibc-ma...

throwaway17_17 7y ago

I think the context you’re working in matters heavily with such an assertion. In fact, it is quite common in game development to bulk allocate and then use custom allocators within these regions specifically avoid the drag on performance and determinism inflicted by malloc.

BubRoss 7y ago
I don't think this confronts anything I said. Bulk allocations with custom allocators would either be exactly what I said (low malloc calls) or would just not use malloc at all and use the system memory mapping functions. The larger point was that an exception being thrown and taking a global lock is not going to tank performance. That would imply that all other threads are trying to take the same lock at the same time and that the thread that gets it holds it for a long time. Even in the case of malloc, where there could be actual heavy lock contention, this is not always a bottleneck.

zabzonk 7y ago

> malloc takes out a global lock Says who? The C Standard says nothing of the kind.

BubRoss 7y ago
All modern common implementations (I think). MSVC's malloc definitely locks. I think gcc, clang and icc do too. This is a big reason why tcmalloc and jemalloc were created.
- gpderetta 7y ago
  I don't know about MSVC, but glibc malloc has per thread caches. At some point you need to hit the global allocator or sbrk or mmap of course and that might take a global lock.

lovecg 7y ago

Of course, it would depend on the workload. An exception here and there wouldn’t matter, but for a somewhat contrived example, using exceptions for something like a bad connection to a database on a server with dozens of threads could easily turn into a concurrency and a resource exhaustion problem (where every thread starts seeing the same error at the same time).

jcelerier 7y ago

> malloc takes out a global lock not anymore for a couple years with glibc : https://www.phoronix.com/scan.php?page=news_item&px=glibc-malloc-thread-cache https://www.phoronix.com/scan.php?page=news_item&px=glibc-ma...