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An advanced version of xv6
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README.md

README.md

jv6

This is the advanced version of xv6.
We support wait-free and fair scheduling system with multi-threading and larger file system than the original xv6.

Dependencies

sudo apt-get install qemu

How to Compile

In jv6 directory,

make clean && make qemu

or in no graphic environment,

make clean && make qemu-nox

How to Test

We provide our test program for newly developed features.

MLFQ & Stride Scheduler

test_master, which has a dependency on test_mlfq, test_stride, test_yield.

LWP

threadtest and threadtest2.

File System

hugefiletest.

MLFQ & Stride Scheduler

Stride Scheduler

The notion, "stride", is a beautiful way to overcome inaccuracy of lottery ticketing. It picks up the process who has the lowest pass so far and advances the pass by stride. I use a linear method for this popping operation, although it can be popped from in priority queue in O(1) and rearranged in O(lg N), because the number of processes (NPROC) is not so large and it might be slower in terms of assembly execution (memory caching or accessing).

Dynamic Allocation

To deal with a problem caused by the dynamic participation of the processes, I will use a variable ss_pass to maintain current pass for schedule. It advances unit strider(stride1) / ss_tickets* per every quantum.

Dynamic Ticket Modification

Ticket, which is inversely proportional to stride, can be modified. An important thing is that ptable.ss_tickets should be less or equal to 80.

MLFQ

As the professor required, 3-level feedback queue is used, time quantum will be proportional to 1:2:4, each level of the queue will adopt RR, and priority boost will be done periodically (100t) to prevent starvation. As doing priority boost, all the processes in every queue of MLFQ would be moved to the highest-level queue. yield() function is always called after ending execution in MLFQ, so that whether the process has fully consumed its given quantum should be checked before. I didn't any linear ADTs but new features in struct proc because neither most ADTs nor dynamic allocation is avoided in kernel implementation.

Boost Up

It is boosted up for every 100 quantum(=timer interrupt) and checked by a formula ticks % BOOSTUP_PERIOD == 0. This is not a lottery because it is continuously checked as ticks increases.


Combine SS with MLFQ

20% vs 80%

Stride Scheduling can be used recursively by considering ticket currencies. Therefore, I will use it for adjusting the proportion between MLFQ and the child stride-scheduler which the project requires. The term "at most" 80% would be kept by following concepts.

ptable.mlfq_stride, ptable.ss_stride - Root SS (competition between schedulers at least 20% vs at most 80%) ptable.ss_pass - expected_pass in Sub-SS (competition between processes), any processes whose ss_ticket is smaller than ss_pass would be chosen by Sub-SS proc.ss_ticket - 1 ticket guarantees 1% of CPU

How to determine the amount of the ticket in SS

Increased y the parameter provided by system call Reduced by exit(), kill()


Loss of precision

We can barely do divisions precisely in the computer. It is encouraged to use a large value for unit_stride to reduce the amount of error, however, it requires complex implementation not related to this class or to install some libraries, which may be not allowed. Therefore, I just used integer, relatively smaller than recommended. It is recommended to set (unsigned) long long.

How if there are no processes?

The professor said that there will be some processes related to kernel. I thought it is also a good concept of having an idle process like Windows but turned out to be needless.

LWP

Definitions

process - initproc or its page directory is not the same as its parent's
thread - its page directory is the same as its parent's

Memory structure

———————
heap
———————
thread stack
..
..
NTHREAD(=16)
———————
process stack
———————
code
———————

Why should we place thread stacks there?

The heap area is allocated by users. For the convenience of managing the heap, we already have malloc() function in umalloc.c. Since umalloc.c is not a part of kernel (you may see Makefile), you cannot allocate the stack of thread once user process gets a control and allocate the heap memory. Therefore you have to reserve area for thread stacks, but it does not mean that you would allocate physical memory for that area.

Now, memory is cheap.

For the memory efficiency, I implemented to allocate and deallocate for every creation or termination of threads, however, it required much more lines and locks. It would make a considerable difference in which threads are frequently made or destroyed, for example, a database system or a webserver. To make it light, I decided to allocate all thread stacks as making process with extra costs of NTHREAD(16) * PGSIZE(4KB) = 64KB and few cycles in a procedure of making process.

We stores related information in ..

The information - which stack has been used or not, virtual memory base of each thread stack - is stored in struct proc only of process.

Functions

fork()

In process, copy all threads. You were enough to check the return value of allocproc() without LWP to know that there are UNUSED proc structure in ptable. But now, you have to check there are as many as its threads, which means you have to store the number of threads of a process and remaining proc structure in ptable.
In thread, copy the calling thread only. It means we do CoW but make only the calling thread active.

exec()

In process, terminate all threads and clear resources. In thread, it becomes child process.

pthread_create()

We now have a concurrency issue with this function. This function can be called by both process and thread.

wait()

If it is called by a process, do as usual. If it is called by a thread, the thread pretends to be process and clean up the other threads and the process.

Changes in proc.c

void exit(void)

It is inevitable not to change* exit() *at all since, in the start_routine, we have to do the callback procedure even though users write not only thread_exit() but "return [retval]" as well. It means that after escaping out from the start_routine, eip(PC) should go to exit().

thread_create()

This function should be alike fork() and exec().
Also, it should allocate separated user stack for a thread. I got it from heap area by using allocuvm()

After allocating the pages, the user stack should be filled with arguments to be immediately executed after switching.
First of all, you have to know that this function will not go to other functions but do return. Thus, the highest word should be filled with arg, and the second highest word should be filled with meaningless value but you should move esp by 2 words ( 8bytes ). After that, simply set ebp as esp, and eip as start_routine like we intended.

The other lines are needed for the same reason as you fork or exec process.

thread_exit()

You should put return value into eax due to xv6 convention.

thread_join()

The process we would like to choose is the one whose tid is the same as thread. However, an adversary can put thread which he does not owe, you should check that the pgdir is not different. The other things are almost same as wait().

Following properties are consistent only within a process, not in a thread.

pgdir, killed, thread, stride_tickets, stride_pass, ofile, cwd, sz.
Thus, I changed all the references to them in sysproc, trap, proc, exec, etc to refresh every changes in a process.

File System

Note that

  1. There exist inode for memory and dinode for disk. inode may have extra information but if you change one you should consider the other to be fit into your system. In this project, a change of the block structure would affect.
  2. The number of blocks per inode, the maximum file size are increased. You may have to change some constant to scale those things.

Things to change

Assuming xv6 codes has good readability and writability, it is clear that the lines I should change would contain NDIRECT. I found the lines containing NDIRECT by cscope and could see a line in file.h and many lines in fs.c, fs.h, and mkfs.c.

size of inode

There are two graceful important assertions that
assert((BSIZE % sizeof(struct dinode)) == 0)
assert((BSIZE % sizeof(struct dirent)) == 0)
To maximize the utility of the given disk space(to reduce extra fragmentation), xv6 forces the size of struct dinode(and inode) to be a power of 2, which means I have to replace an existing direct pointer with a double indirect pointer, or create another 63 pointers to respect the xv6 file system. I thought the former would be much better so changed NDIRECT (=12) to 11 and "uint addrs[NDIRECT+1]" to "uint addrs[NDIRECT+1+1]" in struct inode (in file.h) and struct dinode (in fs.h).

In fs.h, there is only one more thing to be discussed, MAXFILE, which is declared as (NDIRECT + NINDIRECT). I could see that this variable (formally macro) denotes the maximum blocks in inode and added by NINDIRECT * NINDIRECT due to the double indirection inode.

Now we have only fs.c and mkfs.c

There are three functions that refers to NDIRECT variable.

bmap, itrunc, iappend

Thankfully, there already all exist the codes for single indirection. I just copied that and write the code to search one more depth to reach the block.

Panic!

The test program seems to have been working but stoped with panic "out of block". Tracing the frames I found that it is related to sb.size (the size of superblock) set to FSSIZE. The existing file system could endure the large size up to 512000 which is slightly less than 512KB. I calculated maximum block size as below (in Performance) but TA said that a single file would be up to 512KB in the test and recommended to set FSSIZE as 2000 in "3. Tips" . Therefore I set it to 2000 and could pass whole test

Performance

Single Indirection Double Indirection Improved
# of blocks 12(direct) + 512/4(single indirect) = 140 (~70KB) 11 (direct) + 512 / 4 (single indirect) + (512/4) * (512/4) = 16523 (~8.07MB) 1143%
Access time(when full) (12 + 2 * 128)/140 = 1.91 ( 11 + 2 * 128+ 3* 128 * 128) / (11 + 128 + 128 * 128) = 2.99 -56.45%
Space efficiency - Inode proportion( when full ) (140 * 512) /(64+512) = 0.803% (64 + 512 + 512 + 512*128) / ( 11 + 128+ 128 * 128) = 0.787% 1.99%, 0.016%p

Note that access time and Inode proportion for double indirection would be gradually increased/decreased from 1.91 to 2.99 and from 0.803% to 0.787% as the blocks are filled.

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