Hello, When using a=/ and b=swap partitioning scheme on installation or upgrading from 6.3 with same partitioning scheme the default HEAP_SIZE=0xA0000 generates heap full error during boot in 6.4 amd64. To solve this I installed the -stable soiurces from AnonCVS, and changed HEAP_SIZE from 0xA0000 to 0xC0000 (the machine I tested an needs 700687 B so it should work with an HEAP_SIZE=0xB0000) in the file /usr/src/sys/arch/amd64/stand/Makefile.inc . Then I compiled and installed the new bootloader.
dmesg: OpenBSD 6.4 (GENERIC.MP) #364: Thu Oct 11 13:30:23 MDT 2018 dera...@amd64.openbsd.org:/usr/src/sys/arch/amd64/compile/GENERIC.MP real mem = 4142465024 (3950MB) scsibus0 at mpath0: 256 targets mainbus0 at root bios0 at mainbus0: SMBIOS rev. 2.5 @ 0xdc010 (45 entries) bios0: vendor HP version "O15" date 10/25/2010 bios0: HP ProLiant ML110 G5 acpi0 at bios0: rev 2 acpi0: sleep states S0 S1 S4 S5 acpi0: tables DSDT FACP SPMI EINJ HEST BERT SSDT ERST MCFG APIC BOOT SPCR SSDT SSDT SSDT acpi0: wakeup devices USB4(S3) USB5(S3) USB7(S3) ESB2(S4) EXP1(S4) EXP2(S4) EXP3(S4) EXP4(S4) EXP5(S4) EXP6(S4) USB1(S3) US B2(S3) USB3(S3) USB6(S3) ESB1(S3) PCIB(S3) [...] acpitimer0 at acpi0: 3579545 Hz, 24 bits acpimcfg0 at acpi0 acpimcfg0: addr 0xf0000000, bus 0-16 acpimadt0 at acpi0 addr 0xfee00000: PC-AT compat cpu0 at mainbus0: apid 0 (boot processor) cpu0: Intel(R) Xeon(R) CPU X3210 @ 2.13GHz, 2128.29 MHz, 06-0f-0b cpu0: FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CFLUSH,DS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PB E,SSE3,DTES64,MWAIT,DS-CPL,VMX,EST,TM2,SSSE3,CX16,xTPR,PDCM,LONG,LAHF,PERF,SENSOR,MELTDOWN cpu0: 4MB 64b/line 16-way L2 cache cpu0: smt 0, core 0, package 0 mtrr: Pentium Pro MTRR support, 7 var ranges, 88 fixed ranges cpu0: apic clock running at 265MHz cpu0: mwait min=64, max=64, C-substates=0.2, IBE cpu1 at mainbus0: apid 1 (application processor) cpu1: Intel(R) Xeon(R) CPU X3210 @ 2.13GHz, 2128.00 MHz, 06-0f-0b