On Thu, 5 Sep 2019, Huaisheng HS1 Ye wrote: > > -----Original Message----- > > From: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > Sent: Wednesday, September 4, 2019 11:36 PM > > On Wed, 4 Sep 2019, Huaisheng HS1 Ye wrote: > > > > > > > > Hi Mikulas, > > > > > > Thanks for your reply, I see what you mean, but I can't agree with you. > > > > > > For pmem mode, this code path (writecache_flush) is much more hot than > > > SSD mode. Because in the code, the AUTOCOMMIT_BLOCKS_PMEM has been > > > defined to 64, which means if more than 64 blocks have been inserted > > > to cache device, also called uncommitted, writecache_flush would be called. > > > Otherwise, there is a timer callback function will be called every > > > 1000 milliseconds. > > > > > > #define AUTOCOMMIT_BLOCKS_SSD 65536 > > > #define AUTOCOMMIT_BLOCKS_PMEM 64 > > > #define AUTOCOMMIT_MSEC 1000 > > > > > > So when dm-writecache running in working mode, there are continuous > > > WRITE operations has been mapped to writecache_map, writecache_flush > > > will be used much more often than SSD mode. > > > > > > Cheers, > > > Huaisheng Ye > > > > So, you save one instruction cache line for every 64*4096 bytes written to > > persistent memory. > > > > If you insist on it, I can acknowledge it, but I think it is really an > > over-optimization. > > > > Acked-By: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > > Mikulas > > Thanks for your Acked-by, I have learned so much from your code. > > And I have another question about the LRU. > > Current code only put the last written blocks into the front of list > wc->lru, READ hit doesn't affect the position of block in wc->lru. That > is to say, if a block has been written to cache device, even there would > be a lot of READ operation for that block next but without WRITE hit, > which still would flow to the end of wc->lru, and eventually it would be > written back. > > I am not sure whether this behavior disobeys LRU principle or not. But > if this situation above appears, that would lead to some HOT blocks > (without WRITE hit) had been written back, even READ hit many times. Is > it worth submitting patch to adjust the position of blocks when READ > hit? Just a discussion, I want to know your design idea. > > Cheers, > Huaisheng Ye The dm-writecache target is supposed to optimize writes, not reads. Normally, there won't be any reads following a write, because the data would be stored in the cache in RAM. Mikulas -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
