Juergen Keil wrote: > 2009/3/10 Dina <dina.nimeh at sun.com>: >> usr/src/uts/common/io/lofi.c > ... >> Questions: >> >> line 863, 865: lofi_max_comp_cache is tunable? What happens when it >> switches between non-0 and 0 and back again? Is it possible >> for cached segments to get orphaned, or get out of sync? >> (Low likelihood...) >> >> Probably check for lsp->ls_comp_cache NULL-ness and take a >> moment to dump out the cache if any. >> >> What if someone like me tunes lofi_max_comp_cache to 10240? >> Should there be a max lofi_max_comp_cache? > > The lofi_max_comp_cache tunable was intended to compare opensolaris > installation performance with and without the cache, and to compare > performance when caching 1, 2, or more segments. > > It isn't intended to be a tunable that is officially documented. > > When you set it to some large value (10240), you'll be able to shoot > yourself into the foot - the kernel might run out of kernel heap. > > > Changing the tunable at runtime shouldn't break anything or leak memory. > At this time you can increase the value at runtime, and the code should > "do the right thing". Decreasing / changing from non-0 to 0 doesn't return > cache kmem to the system immediately - it does release the memory when > you unmap the file.
This is the paragraph I wanted to focus on. If you have it set to 1 at start, then during runtime you set it to 0. Now you have no caching. The cache memory isn't returned immediately, yes. It sits there. <insert unknown period of time with unknown events w/r/t cached segment> Now you turn it back on by setting the value back to 1. The segment that is already cached is still sitting there. What happens to it? It releases the memory at unmap time, is unmapping the file a guaranteed event at this point? If not, then does this lead to a stale segment in the cache? > > Do we really need code to shrink the lofi decompress cache at runtime? I think I understand that 1 cached segment gives a nice boost. And that the improvement is probably not linear. At some point it doesn't matter if you have 5 or 10 for your max cache size, the incremental performance improvement may no longer be noticeable. That's why I asked what would happen if I set it 10240. Would it really grow to 10240, or would it cut me off somewhere and not grow anymore, to prevent running out of heap? D.
