Minimum Limit For Cache Allocation?

[aaditya2200]

Hi,

I have been using cachelib to build a swap space. I want the cache to be of a certain size based on an input parameter, and I have designed it as follows:

cache::cache(backing_store *bs, uint64_t n)
: backstore(bs), max_in_memory_objects(n) {
auto itemDestructor =
[&](const facebook::cachelib::LruAllocator::DestructorData &data) {
write_back(data.item);
};
Cache::Config config;
config.setCacheSize(1 * sizeof(object) * n)
.setCacheName("Lru_nodes")
.setAccessConfig({25, 10})
.setItemDestructor(itemDestructor)
.validate();
gcache_ = std::make_unique(config);
default_pool = gcache_->addPool("default_pool",
gcache_->getCacheMemoryStats().ramCacheSize);
}

n here is the cache size as an input parameter. What I have noticed is that for small values of n, the allocation fails with the error

E20240526 08:58:46.625504 68212 ExceptionTracer.cpp:222] exception stack complete
terminate called after throwing an instance of 'std::invalid_argument'
what(): not enough memory for slabs

The minimum n for which I can get an allocation seems to be 150K. I was wondering if there is any lower bound to the amount of memory we can allocate?

[therealgymmy]

cachelib uses slab allocator underneath. The cache size needs to be a multiple of 4MBs. (Tho you don't have to specify the exact number we'll internally round down to 4MB-aligned size). Anyway that means you need to make the cache at least 2 Slabs. (we use 1 slab for internal metadata for other slabs).

Typically I would suggest just make the cache 10 slabs or 100 slabs for testing.