On Fri 24 Apr 2015 03:04:06 PM CEST, Kevin Wolf <kw...@redhat.com> wrote:
>> >> I think it would be nice to have a way to free unused cache >> >> entries after a while. >> > >> > Do you think mmap plus a periodic timer would work? >> > >> > I'm hesitant about changes like this because they make QEMU more >> > complex, slow down the guest, and make the memory footprint >> > volatile. But if a simple solution addresses the problem, then I'd >> > be happy. >> >> I understand. One possible approach would be to use the timer only if >> the cache size requested by the user is large, so >> >> 1) we only try to save memory when the amount of memory to save is >> significant. >> 2) we don't make the default scenario slower. >> >> I'll try to see if I can come up with a good solution (and with more >> numbers). > > I suspect that at this point, Anthony would have reminded us about > separation of policy and mechanism. The very least we need is some > option to control the policy (probably defaulting to no automatic > cache size changes). There's also the problem that with a single chunk of memory for all cache tables it's not so easy to free individual entries. One possible solution would be madvise() with MADV_DONTNEED, and that's rather simple to use, but I'm unsure about its portability. > The other option would be to let the management tool change the cache > size options at runtime (and as it happens, my current bdrv_reopen() > overhaul allows just that - except that a QMP interface isn't planned > yet), but I'm not sure if it has enough information to make good > decisions. If we know what the criteria are, they could possibly be > exposed, though. But what would you do there? Recreate the whole cache? Berto
