BH/Quasar: Stall On Dest Reads - Impact And Concerns

Introduction to Blackhole & Quasar Stall Behavior

In the realm of hardware optimization, Blackhole and Quasar offer a unique capability to fine-tune the behavior of TTI_STALLWAIT functions. Traditionally, these functions, designed to stall for specific events, would wait for the packer engine to become completely idle. However, a more nuanced approach is now possible: stalling specifically on the completion of destination reads. This enhancement, controlled by a chicken bit, promises performance improvements by reducing unnecessary wait times. Let's delve into the specifics of this feature, its benefits, and the potential concerns it raises.

The ability to stall specifically on destination reads, rather than waiting for the entire packer engine to idle, stems from the observation that often the primary bottleneck is the destination bank's readiness for new math outputs. By focusing the stall on this specific event, we can potentially unlock significant performance gains. The implementation revolves around a configurable bit, aptly named stallwait_waits_just_for_pack_dest_read, which governs whether the STALLWAIT function waits for complete packer idleness or just the completion of destination reads. This chicken bit acts as a toggle, allowing developers to switch between the two stall behaviors based on the specific needs of their application. The rationale behind this feature lies in the common scenario where we're primarily waiting for the destination bank to be ready for new math outputs. In such cases, waiting for the entire packer engine to idle is an overkill, leading to wasted cycles. By enabling stallwait_waits_just_for_pack_dest_read, the STALLWAIT function becomes more efficient, stalling only until the destination reads are finished. This targeted stalling mechanism reduces latency and improves overall throughput, especially in workloads where destination bank contention is a major performance bottleneck. However, this optimization comes with a caveat: the potential impact on other operations that might genuinely require a stall on the entire packer engine.

The Rationale Behind Stall on Dest Reads

The primary motivation for implementing this feature is to optimize scenarios where the destination register bank needs to be cleared before it can be reused by the math thread. Current designs often require a stall on the packer to ensure that the destination is ready, as illustrated in the code snippet below:

template <DstSync Dst, bool is_fp32_dest_acc_en = false>
inline void _llk_pack_dest_section_done_() {
#ifdef PERF_DUMP
    if constexpr (MATH_PACK_DECOUPLE) {
        return;
    }
#endif

    constexpr bool clear_dest = (Dst != DstSync::SyncTile16);

    if constexpr (clear_dest){
        TTI_STALLWAIT(p_stall::STALL_MATH, p_stall::PACK);  // wait for pack to finish

        if constexpr (Dst == DstSync::SyncFull) {
            constexpr uint32_t CLEAR_MODE = is_fp32_dest_acc_en ? p_zeroacc::CLR_ALL_32B : p_zeroacc::CLR_ALL;
            TT_ZEROACC(CLEAR_MODE, ADDR_MOD_1, 0);
        } else {
            static_assert((Dst == DstSync::SyncHalf) || (Dst == DstSync::SyncTile2));
            constexpr uint32_t CLEAR_MODE = is_fp32_dest_acc_en ? p_zeroacc::CLR_HALF_32B : p_zeroacc::CLR_HALF;
            TT_ZEROACC(CLEAR_MODE, ADDR_MOD_1, (dest_offset_id) % 2);
        }
    }
}

In this code, the TTI_STALLWAIT function is used to wait for the packer to finish before clearing the destination. By enabling the chicken bit, this stall can be made more efficient by only waiting for the destination reads to complete. The stall on the packer ensures that the destination register bank is fully available before it is cleared and made ready for the next math operation. Without this stall, there's a risk of data corruption or incorrect results due to the math thread attempting to use the destination before it has been properly prepared. The TTI_STALLWAIT function, in essence, acts as a synchronization point between the packer and the math thread. The current implementation of TTI_STALLWAIT waits for the packer to completely finish its operation. However, the optimization offered by the chicken bit acknowledges that often, the most critical aspect is the completion of destination reads. Therefore, by enabling stallwait_waits_just_for_pack_dest_read, the stall can be shortened, leading to improved performance. The code snippet clearly demonstrates the necessity of stalling before clearing the destination. The stall ensures that the packer has finished writing to the destination, preventing any potential conflicts or data inconsistencies. By optimizing this stall, we can reduce the overall execution time and improve the efficiency of the system.

Concerns and Potential Drawbacks

While the potential performance gains are attractive, there's a valid concern: if this chicken bit is enabled, all stalls on the packer will be modified to only stall on destination reads. This raises the question of whether there are other operations that genuinely require a stall on the entire packer engine. If such operations exist, enabling this bit could inadvertently introduce performance regressions or even functional errors. It is imperative to carefully analyze all use cases of packer stalls to ensure that this change does not negatively impact other parts of the system. The central concern revolves around the global nature of the chicken bit. Its activation affects all stalls on the packer, potentially overlooking specific instances where a complete packer stall is necessary. For example, there might be operations that rely on the packer engine being in a fully idle state before proceeding. By switching to a destination-read-only stall, these operations could be prematurely triggered, leading to unpredictable behavior. Therefore, a thorough investigation is required to identify all packer stall use cases and assess their sensitivity to this change. Ideally, a more granular control mechanism would be desirable, allowing developers to specify the stall behavior on a case-by-case basis. However, in the absence of such fine-grained control, a cautious approach is warranted. Before enabling the chicken bit, it's crucial to conduct extensive testing and performance profiling to ensure that the change yields overall improvements without compromising the functionality or performance of other components.

Analyzing the Impact of the Chicken Bit

To fully understand the implications of enabling the stallwait_waits_just_for_pack_dest_read chicken bit, a comprehensive analysis of all packer stall use cases is essential. This analysis should identify any operations that rely on the packer engine being completely idle and assess the potential impact of switching to a destination-read-only stall. Furthermore, performance benchmarks should be conducted to quantify the actual performance gains achieved by enabling the bit, as well as any potential regressions in other areas. The analysis should also consider the frequency and criticality of different packer stall use cases. If a particular use case that requires a full packer stall is rarely executed or has a minimal impact on overall performance, then the risk of enabling the chicken bit might be acceptable. However, if such a use case is frequently executed or is critical for system functionality, then enabling the bit might not be a viable option. In addition to performance considerations, it's also important to assess the potential impact on system stability and reliability. A destination-read-only stall might introduce race conditions or timing issues that could lead to unpredictable behavior or even system crashes. Therefore, rigorous testing and validation are crucial to ensure that the change does not compromise the robustness of the system. The analysis should also take into account the potential for future changes in the system architecture or software stack. If new operations are introduced that rely on a full packer stall, then the chicken bit might need to be disabled, potentially negating any performance gains achieved in the past. Therefore, a long-term perspective is essential when evaluating the impact of enabling the bit.

Conclusion: A Cautious Approach to Optimization

The stallwait_waits_just_for_pack_dest_read chicken bit offers a promising avenue for performance optimization in Blackhole and Quasar. By selectively stalling on destination reads, we can potentially reduce unnecessary wait times and improve overall throughput. However, this optimization comes with a risk: the potential impact on other operations that might require a full stall on the packer engine. Therefore, a cautious and data-driven approach is warranted. Before enabling this chicken bit, it is crucial to conduct a thorough analysis of all packer stall use cases, quantify the potential performance gains, and assess any potential regressions or stability issues. Only with a comprehensive understanding of the implications can we make an informed decision about whether to enable this optimization. In conclusion, the decision to enable the stallwait_waits_just_for_pack_dest_read chicken bit should be based on a careful evaluation of the trade-offs between performance gains and potential risks. A data-driven approach, coupled with rigorous testing and validation, is essential to ensure that this optimization truly benefits the system as a whole. Remember to stay informed and continue exploring the depths of hardware optimization! More information on hardware acceleration can be found at Cloudflare Hardware Acceleration