VarsizeBlkAllocator crash when running with smaller chunk size.

[xiaoxichen]

    for (const auto& slab_cfg : cfg.m_per_slab_cfg) {
        std::vector< blk_num_t > level_limits;
        level_limits.reserve(slab_cfg.m_level_distribution_pct.size());
#ifndef NDEBUG
        HS_DBG_ASSERT_EQ(slab_cfg.slab_size, slab_size, "Slab config size is not contiguous power of 2");
        slab_size *= 2;
#endif

        blk_num_t sum{0};
        for (const auto& p : slab_cfg.m_level_distribution_pct) {
            const blk_num_t limit{static_cast< blk_num_t >((static_cast< double >(slab_cfg.max_entries) * p) / 100.0)};
            sum += limit;
            level_limits.push_back(limit);
        }
        HS_DBG_ASSERT_GE(slab_cfg.max_entries, sum, "Slab config distribution does not add to 100%");
        if (sum < slab_cfg.max_entries) {
            level_limits[0] += slab_cfg.max_entries - sum; // Put the remaining to the first priority
        }

        auto ptr{std::make_unique< SlabCacheQueue >(slab_cfg.slab_size, level_limits, slab_cfg.refill_threshold_pct,
                                                    metrics)};
        m_slab_queues.push_back(std::move(ptr));

It is possible we have a level with 0 limit, which will crash in constructing SlabCacheQueue

unknown file: Failure
C++ exception with description "MPMCQueue with explicit capacity 0 is impossible" thrown in the test body.

$43 = std::vector of length 2, capacity 2 = {2, 0}
(gdb) p slab_cfg.m_level_distribution_pct
$44 = std::vector of length 2, capacity 2 = {70, 30}
(gdb) p slab_cfg.max_entries
$45 = 2
(gdb) p slab_cfg.max_entries
$46 = 2
(gdb) p slab_cfg
$47 = (const homestore::SlabCacheConfig::_slab_config &) @0x617000069cf8: {slab_size = 128, max_entries = 2, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}
(gdb) p cfg.m_per_slab_cfg
$48 = std::vector of length 9, capacity 9 = {{slab_size = 1, max_entries = 707, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 2, max_entries = 118, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 4, max_entries = 59,
    refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 8, max_entries = 25, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 16, max_entries = 21, refill_threshold_pct = 60,
    m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 32, max_entries = 10, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 64, max_entries = 5, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30},
    m_name = "varsize_chunk_0"}, {slab_size = 128, max_entries = 2, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 256, max_entries = 3, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}}

[xiaoxichen]

Thread 1 "homestore_test" hit Breakpoint 1, homestore::VarsizeBlkAllocator::VarsizeBlkAllocator (this=0x61500005d290, cfg=..., is_fresh=true, chunk_id=0) at /var/home/centos/.conan/data/homestore/6.2.3-49/oss/master/build/98a6c2ddffe4d1fb00eb495a8407dc88f8f61572/src/lib/blkalloc/varsize_blk_allocator.cpp:53
53	VarsizeBlkAllocator::VarsizeBlkAllocator(VarsizeBlkAllocConfig const& cfg, bool is_fresh, chunk_num_t chunk_id) :
(gdb) p cfg
$1 = (const homestore::VarsizeBlkAllocConfig &) @0x7fffffff9480: {<homestore::BlkAllocConfig> = {_vptr.BlkAllocConfig = 0x55555e0080c0 <vtable for homestore::VarsizeBlkAllocConfig+16>, m_blk_size = 4096, m_align_size = 512, m_capacity = 4224, m_blks_per_portion = 4224, m_persistent = false, m_unique_name = "varsize_chunk_0"}, m_phys_page_size = 4096, m_nsegments = 1, m_blks_per_temp_group = 4224,
  m_max_cache_blks = 3379, m_slab_config = {m_name = "varsize_chunk_0", m_per_slab_cfg = std::vector of length 9, capacity 16 = {{slab_size = 1, max_entries = 707, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 2, max_entries = 118, refill_threshold_pct = 60,
        m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 4, max_entries = 59, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 8, max_entries = 25, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30},
        m_name = "varsize_chunk_0"}, {slab_size = 16, max_entries = 21, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 32, max_entries = 10, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 64, max_entries = 5,
        refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 128, max_entries = 2, refill_threshold_pct = 60, m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}, {slab_size = 256, max_entries = 3, refill_threshold_pct = 60,
        m_level_distribution_pct = std::vector of length 2, capacity 2 = {70, 30}, m_name = "varsize_chunk_0"}}}, m_use_slabs = true}