Re: [PATCH v1 4/4] accel/tcg: increase default code gen buffer size for 64 bit

[Alex Bennée]

Richard Henderson <address@hidden> writes:

> On 2/26/20 10:10 AM, Alex Bennée wrote:
>> While 32mb is certainly usable a full system boot ends up flushing the
>> codegen buffer nearly 100 times. Increase the default on 64 bit hosts
>> to take advantage of all that spare memory. After this change I can
>> boot my tests system without any TB flushes.
>
>> +#if TCG_TARGET_REG_BITS == 32
>>  #define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (32 * MiB)
>> +#else
>> +#define DEFAULT_CODE_GEN_BUFFER_SIZE_1 (2 * GiB)
>> +#endif
>
> This particular number, I'm not so sure about.
>
> It makes sense for a lone vm, running in system mode, on a large-ish host.
> It's more questionable for a large-ish host running many system mode vm's,
> although one can tune that from the command-line, so perhaps it's
> still ok.

Yeah it would be nice to get some feedback from users. I suspect system
emulation means the mmap is less efficient due to the sharding of the
translation buffer.

> It does not make sense for a linux-user chroot, running make -jN, on just 
> about
> any host.  For linux-user, I could be happy with a modest increase, but not 
> all
> the way out to 2GiB.
>
> Discuss.

Does it matter that much? Surely for small programs the kernel just
never pages in the used portions of the mmap?

That said does linux-user have a better idea of the size of the problem
set before we start running? Could we defer calling tcg_exec_init until
we have mapped in the main executable and then size based on that?

>
>
> r~


-- 
Alex Bennée