"Also with more ram, try 48": A Pivot Point in the Flash-Attention Build Saga

In the middle of a grueling session to set up a machine learning environment on a remote Ubuntu 24.04 server, the user issues a message that is deceptively simple but carries immense weight. The message reads:

Also with more ram, try 48

This is message [msg 46] in the conversation, and it arrives at a critical juncture. The session has been consumed for many rounds by a single, stubborn problem: building the flash-attn library from source. The user and the assistant have been locked in a collaborative debugging loop, iterating on a single parameter — MAX_JOBS, which controls the number of parallel compilation processes — as the system repeatedly runs out of memory and crashes. This message marks the moment when the hardware itself is upgraded to break the deadlock.

The Context: A Build That Keeps OOMing

To understand why this message matters, one must appreciate what led to it. Earlier in the session, the assistant successfully installed NVIDIA drivers (version 590.48.01) and CUDA Toolkit 13.1 on a machine with two (later eight) NVIDIA RTX PRO 6000 Blackwell GPUs ([msg 7]–[msg 12]). A Python virtual environment was created using uv, and core ML packages like PyTorch were installed ([msg 16]). But when it came time to install flash-attn — a highly optimized attention mechanism kernel that must be compiled from source when no pre-built wheel matches the exact CUDA and PyTorch versions — everything ground to a halt.

The build process for flash-attn is exceptionally memory-intensive. It compiles dozens of CUDA kernel variants in parallel using nvcc and ptxas (the PTX assembler). Each compilation process can consume gigabytes of RAM. The assistant initially attempted MAX_JOBS=128 ([msg 29]), leveraging the machine's 128 CPU cores. The system promptly ran out of memory. The user responded with "OOMing, try 64" ([msg 36]). That also failed. "Still oom, go 32" ([msg 39]–[msg 40]). Even 32 parallel jobs caused the machine to become completely unresponsive — the assistant's attempts to kill the runaway processes timed out ([msg 41]–[msg 42]), and the assistant noted, "Machine is probably thrashing" ([msg 43]).

The situation was dire. The build had pushed the system past its limits, and the remote machine was effectively deadlocked, unable to even execute kill commands. The user's solution was decisive: "rebooted" ([msg 44]). The assistant confirmed with an empty bash call ([msg 45]), ready to resume.

What the Message Actually Says

The user's message, "Also with more ram, try 48," contains two distinct pieces of information delivered in a single, terse sentence.

"Also with more ram" reveals that during the reboot, the machine was physically upgraded with additional RAM. This is not a configuration change or a swap file adjustment — the user is referring to a hardware modification. The machine went from having an unknown (but insufficient) amount of RAM to having 432 GiB, as confirmed in the assistant's very next message ([msg 47]): "Mem: 432Gi." This is an extraordinary development in a coding session. The debugging process didn't just involve changing software parameters; it escalated to the point where the physical hardware was modified to solve the problem. The user, who has physical access to the machine, took the opportunity during the forced reboot to install more memory.

"try 48" proposes a new value for MAX_JOBS. This is a deliberate choice, not a random number. The previous attempts were 128, 64, and 32. All failed. The user is now suggesting 48 — a value that splits the difference between the failed 64 and the also-failed 32. But why 48 specifically? The reasoning appears to be: if 64 caused OOM and 32 also caused OOM (on the old RAM configuration), then with more RAM available, a value between them might be the sweet spot. The user is implicitly assuming that the OOM at 32 was still due to insufficient total RAM, and that with the upgrade, even 48 might be feasible. It's a hypothesis grounded in the new hardware reality.

Assumptions and Reasoning

The user's message rests on several assumptions. First, that the OOM failures were purely a function of total system RAM rather than some other bottleneck like per-process memory limits, swap configuration, or kernel memory fragmentation. Second, that doubling (or significantly increasing) the RAM would allow proportionally more parallel compilation jobs. Third, that the assistant would understand the shorthand "try 48" to mean setting MAX_JOBS=48 in the build command, continuing the same pattern established over the previous rounds.

These assumptions turned out to be correct. In the following round ([msg 47]), the assistant verified the new RAM total (432 GiB), reloaded the NVIDIA driver, confirmed the GPUs were still operational, and then proceeded with the build. The message "Also with more ram, try 48" functioned as both a status update and a directive, compressed into seven words.

The Thinking Process Visible in This Message

What makes this message remarkable is what it reveals about the user's thinking process. The user is not just blindly guessing numbers. There is a clear pattern of systematic trial and error:

  1. Start aggressive: MAX_JOBS=128 (leverage all cores)
  2. Upon failure, halve: MAX_JOBS=64
  3. Upon failure, halve again: MAX_JOBS=32
  4. Upon failure (even 32 OOMs), recognize that the hardware is the bottleneck
  5. Upgrade the hardware (reboot with more RAM)
  6. Propose a value between the two most recent failures: MAX_JOBS=48 The choice of 48 is particularly interesting. It is not a power of two (32, 64, 128), breaking the halving pattern. It suggests the user is thinking: "32 failed before, but that was with less RAM. 64 also failed before. With more RAM, let's try something in the middle — 48 — which is 50% more than 32 but 25% less than 64." This is heuristic reasoning, not derived from any formula or documentation. It reflects an intuitive sense that the RAM constraint was the limiting factor and that the upgrade provides headroom for roughly 50% more parallelism.

Input and Output Knowledge

To understand this message, one needs to know: what MAX_JOBS controls in the context of building flash-attn (the number of parallel compilation processes); the history of failed attempts at 128, 64, and 32; the fact that the machine was rebooted due to OOM; and the implicit understanding that "with more ram" refers to a physical hardware upgrade performed during the reboot.

The message creates new knowledge: the assistant now knows that the machine has been upgraded, that the user wants to try MAX_JOBS=48, and that the user is actively engaged in the debugging process and willing to modify hardware to solve software problems. It also establishes a new baseline for the build attempt — one that, as it turns out, will succeed.

A Turning Point

This message is the turning point in the flash-attn installation saga. After this, the build succeeds, and the session moves on to installing vLLM and deploying the GLM-5-NVFP4 model. The message encapsulates the unique dynamic of this session: a human and AI working together, the human providing physical-world intervention (adding RAM, rebooting) while the AI handles the software configuration, all communicating through a shared vocabulary of build parameters and error patterns. It is a small message — seven words — but it represents the culmination of a long debugging loop and the moment the bottleneck was finally broken.