Memory limits

Posted Sep 8, 2025 7:50 UTC (Mon) by arnd (subscriber, #8866)
In reply to: Missing the reason by josh
Parent article: No more 32-bit Firefox support

Modern Firefox is likely to hit both the physical (installed) memory limits and the virtual addressing limits of 32-bit machines, and it really only gets worse over time:

On arm32 and x86-32 systems running a kernel config with HIGHMEM and VMSPLIT_3G, the per process address limit is 3GB, as the 32-bit address space is shared with the kernel's lowmem (768MB) and vmalloc (256MB) areas.
Physical memory on x86-32 hardware is theoretically limited to 4GB with CONFIG_HIGHMEM_4G, but in practice the chipsets have a 3GB limit at best. 32-bit arm systems can theoretically go up to 16GB with HIGHMEM and LPAE, but most SoCs cannot connect more than 2GB (four 256Mb x16 DDR3 chips).
The 3GB virtual address limit will typically get lowered to 1.75GB or 2GB once the kernel loses support for highmem, because one has to use CONFIG_VMSPLIT_2G to keep accessing 2GB the physical memory (lowmem). 32-bit systems with 3GB or more RAM at that point lose both physical memory and virtual address limits.
On 64-bit systems, the virtual addressing can go up to 4GB for 32-bit compat tasks, so there may be cases where a particular 32-bit browser binary works fine when tested on 64-bit systems, but is unable to fit within the virtual limits on 32-bit kernels with the same amount of physical memory.
Both browser implementations and popular websites tend to use more memory every year, from a combination of added features and developers caring less about low-end devices. 3GB of physical memory is already tight for opening multiple tabs, while 3GB of address space may not be enough for a process showing a website with a lot of complex Javascript.
Mozilla is pushing integrated AI features into their browsers, which likely brings a lot of workloads over one of the above limits, making it completely unusable.