[RFC PATCH 2/8] accel/tcg: enlarge the TB jump cache to 64K entries

[Matt Turner <mattst88@gmail.com>]

The per-CPU TB jump cache has held 4096 entries since it was introduced.
That is too small for guests running large programs: an emulated compiler
misses often enough that the fallback qht lookup shows up prominently in
a profile.

Measured with qemu-alpha running an emulated alpha gcc 16.2.0 compiling
the SQLite 3.45.1 amalgamation (255k lines, -O2) on an x86-64 host. The
compile performs 34.2 billion TB executions, of which 8.4 billion take
the indirect dispatch path.

Sizing curve, on top of the preceding patch, instructions retired and
wall clock:

    12 bits (  64 KiB): 1,563,829,403,943          133.13s
    14 bits ( 256 KiB): 1,493,865,985,972  -4.47%  124.89s  -6.19%
    16 bits (   1 MiB): 1,469,729,281,442  -6.02%  120.97s  -9.13%
    18 bits (   4 MiB): 1,462,262,363,257  -6.49%  120.16s  -9.74%

16 bits is the knee. 18 buys another 0.47% of instructions for four times
the memory, and since instructions retired does not account for the data
cache pressure of a 4 MiB table, that 0.47% is probably not real: the
wall clock difference between 16 and 18 bits is 0.67%, against a
run-to-run spread of the same order.

In a perf profile the mechanism is visible directly: tb_htable_lookup(),
which is where qht_lookup_custom() lands once it is inlined in an LTO
build, falls from 5.73% of samples to 1.52%.

The cost is memory: the cache grows from 64 KiB to 1 MiB per vCPU. That
is easy to justify for a single-vCPU linux-user process and less obvious
for system emulation with many vCPUs, so this may want to be sized by
target or made tunable rather than raised unconditionally.

Signed-off-by: Matt Turner <mattst88@gmail.com>
---
 accel/tcg/tb-jmp-cache.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git ./accel/tcg/tb-jmp-cache.h ./accel/tcg/tb-jmp-cache.h
index c3a505e394..268dacd7ba 100644
--- ./accel/tcg/tb-jmp-cache.h
+++ ./accel/tcg/tb-jmp-cache.h
@@ -12,7 +12,7 @@
 #include "qemu/rcu.h"
 #include "exec/cpu-common.h"
 
-#define TB_JMP_CACHE_BITS 12
+#define TB_JMP_CACHE_BITS 16
 #define TB_JMP_CACHE_SIZE (1 << TB_JMP_CACHE_BITS)
 
 /*
-- 
2.54.0